Import existing Claude/Codex/OpenCode sessions (#766)

* Restrict provider import fan-out to enabled, non-derived allowlist

Move provider eligibility from a session-level post-filter to the agent
manager's fan-out so disabled providers are never spawned (fixes the
Gemini-disabled hang) and derived providers (like a custom Z.AI extending
Claude) don't duplicate their base provider's sessions.

Add an IMPORTABLE_PROVIDERS allowlist (claude, codex, opencode); ACP-based
providers are excluded from import discovery entirely.

* Filter Codex persisted threads by cwd before hydrating turns

Codex's listPersistedAgents previously called thread/read includeTurns on
every thread returned by thread/list, even when the import sheet was
scoped to one cwd. With many recent threads outside the scope, that fan
of reads dominated the import-sheet open latency.

Add an optional cwd hint to ListPersistedAgentsOptions, thread it through
the agent manager, and have Codex pre-filter the cheap thread/list rows by
cwd before scheduling the per-thread hydration. Widen the thread/list
window when filtering since most rows will be from other cwds. Other
providers ignore the hint.

* Fan out import session fetch per enabled provider

Sheet now reads the providers snapshot already warmed by the workspace
screen, filters to enabled importable providers (claude/codex/opencode),
and issues one fetchRecentProviderSessions request per provider via
useQueries with limit 15. Results are merged, deduped by handle, and
sorted by last activity.

Falls back to a single unfiltered request when the daemon does not
expose a provider snapshot (older servers). Per-provider failures show
inline so a single broken provider does not hide the rest.

* Add provider filter badges to import sheet

Show "All" plus one badge per enabled importable provider above the
session list when more than one is available. Single-select narrows the
already-merged list — no refetch. Each row now shows the provider's
icon next to its label so multiple providers stay visually distinct.

Selection resets when the sheet closes.

* Distinguish already-imported empty state in import sheet

Track how many descriptors were dropped because their handle was
already imported, surface the count back to the client as an optional
`filteredAlreadyImportedCount`, and switch the sheet copy to
"All recent sessions are already imported." when the merged list is
empty solely due to that filter.

* Cover already-imported provider session filtering

* List persisted OpenCode sessions for import

* Revalidate import sessions when sheet reopens

* Add agent persistSession plumbing

* Drop redundant visibility-transition refetch and test boilerplate

The import sheet's queries are gated on `enabled: visible`, and react-query
already refetches stale data when `enabled` flips on, so the manual
`wasVisibleRef` + `invalidateQueries` effect was redundant. Collapse the
two `selectedProvider` reset effects into one.

Drop `expect(...).toBeTruthy()` after `findByText` / `getByText` — those
helpers throw on miss, so the assertion was noise.

Drop `| null` from `buildOpenCodePersistedAgentDescriptor`'s return type
(it never returns null) and the dead `Boolean(descriptor)` filter at the
caller.

* Add provider-opaque session import path

Introduces fetchRecentProviderSessions on the daemon client plus an
importAgent overload accepting providerId/providerHandleId, and a
projection helper that renders persisted descriptors as provider-opaque
recent-session payloads with prompt previews.

* Unslop the import-session feature

Centralizes the importable-providers list, normalizes import_agent_request
shape at one boundary, and removes the per-provider fan-out fallback in
the import sheet. Old daemons without features.providersSnapshot now show
"Update host" instead of being served a degraded path.

Replaces internal-helper unit tests on the OpenCode and Codex providers
with boundary tests through listPersistedAgents; makes storageRoot
injectable on the OpenCode provider so the boundary test can target a
temp directory.

Splits the CLAUDE.md back-compat rule into two contracts: protocol stays
backward-compatible (always); features may require new daemons (no
fallback paths). Tags the two new compat shims with COMPAT(name) and
target removal dates.

* Extract importable-session listing into its own module

Move listImportableProviderSessions out of session.ts and drop the
unused generic listPersistedAgents from AgentManager.

CLAUDE.md

@@ -75,12 +75,19 @@ See [docs/development.md](docs/development.md) for full setup, build sync requir
 - **Always use npm scripts for linting and formatting.** Do not run tools directly with `npx eslint`, `npx oxfmt`, `npx oxlint`, or package-local binaries. For targeted checks, pass file paths through the npm script:
   - `npm run lint -- packages/app/src/components/message.tsx`
   - `npm run format:files -- CLAUDE.md packages/app/src/components/message.tsx`
-- **NEVER make breaking changes to WebSocket or message schemas.** The primary compatibility path is old mobile app clients talking to newly updated daemons. Users update desktop and daemon first, then keep running the old app for a while. Every schema change MUST be backward-compatible for old clients against new daemons:
-  - New fields: always `.optional()` with a sensible default or `.transform()` fallback.
-  - Never change a field from optional to required.
-  - Never remove a field — deprecate it (keep accepting it, stop sending it).
-  - Never narrow a field's type (e.g. `string` → `enum`, `nullable` → non-null).
-  - Test with: "does a 6-month-old client still parse this?" and "does a 6-month-old daemon still send something this client accepts?"
+- **The protocol stays backward-compatible. Features don't have to.** Two separate contracts:
+  - **Protocol contract (always):** schema changes must not break parsing in either direction. An old client must still parse messages from a new daemon; a new daemon must still parse messages from an old client.
+    - New fields: `.optional()` with a sensible default or `.transform()` fallback.
+    - Never flip optional → required, remove fields, or narrow types (`string` → `enum`, `nullable` → non-null).
+    - Removed fields stay accepted (we stop sending them, not stop reading them).
+    - Test with: "does a 6-month-old client still parse this?" and "does a 6-month-old daemon still send something this client accepts?"
+  - **Feature contract (per-feature):** a new feature may require a new daemon capability. The client detects whether the capability is present and either runs the feature or shows "Update the host to use this." That's it.
+    - **No fallback paths.** Don't write a degraded version of a new feature that runs on old daemons. Don't fan out across legacy RPCs to simulate a missing capability. The user upgrades or doesn't get the feature.
+    - **No defensive branches scattered through the feature.** Capability detection happens in one place; downstream code reads a clean shape.
+    - **Capability flags live in `server_info.features.*`** with a single `// COMPAT(featureName): added in v0.1.X, drop the gate when floor >= v0.1.X` comment marking the cleanup site.
+    - Existing functionality keeps working across versions — that's the protocol contract doing its job. New-feature degradation is not the goal.
+
+- **All back-compat shims are tagged and dated for cleanup.** Every shim that exists for old-client/old-daemon support carries a `COMPAT(name)` comment with the version it was added in and a target removal date (typically 6 months out). One grep — `rg "COMPAT\("` — should produce the full list of cleanup work. Don't bury back-compat in untagged `??`-fallbacks or optional-chain tunnels — that's how it stops being deletable.
 
 ## Platform gating