feat: project-level prompts for metadata generation (#836)

* feat(server): project-level prompts for metadata generation

Add `metadataGeneration` config to `paseo.json` so projects can customize the
four LLM-generated metadata strings — agent title, worktree branch name, commit
message, and pull request title+body. Each entry takes an `instructions` string
that gets injected as a `<user-instructions>` block between the default rules
and the load-bearing JSON format contract, with wrapper text saying user
instructions override defaults. Missing/empty/invalid config keeps every prompt
byte-identical to today.

* refactor(server): centralize project metadata prompt building

Three near-identical readers and four gate-then-wrap blocks collapsed
into one buildMetadataPrompt helper. wrapWithUserInstructions tightened
to require a non-empty string; its misleading empty fallback is gone.

* feat(app): edit metadata generation prompts in project settings

Adds four textareas (agent title, branch name, commit message, pull
request) under project settings, modeled on the existing setup/teardown
pattern. Round-trips through paseo.json preserving unknown sibling
fields at both the metadataGeneration and entry level.

packages/server/src/shared/messages.ts

@@ -55,22 +55,30 @@ import {
 import {
   PaseoConfigRawSchema,
   PaseoLifecycleCommandRawSchema,
+  PaseoMetadataGenerationEntrySchema,
+  PaseoMetadataGenerationSchema,
   PaseoScriptEntryRawSchema,
   PaseoWorktreeConfigRawSchema,
   PaseoConfigRevisionSchema,
   ProjectConfigRpcErrorSchema,
   type PaseoConfigRaw,
   type PaseoConfigRevision,
+  type PaseoMetadataGeneration,
+  type PaseoMetadataGenerationEntry,
   type PaseoScriptEntryRaw,
   type ProjectConfigRpcError,
 } from "../utils/paseo-config-schema.js";
 export {
   PaseoConfigRawSchema,
   PaseoLifecycleCommandRawSchema,
+  PaseoMetadataGenerationEntrySchema,
+  PaseoMetadataGenerationSchema,
   PaseoScriptEntryRawSchema,
   PaseoWorktreeConfigRawSchema,
   type PaseoConfigRaw,
   type PaseoConfigRevision,
+  type PaseoMetadataGeneration,
+  type PaseoMetadataGenerationEntry,
   type PaseoScriptEntryRaw,
   type ProjectConfigRpcError,
 };