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/utils/wrap-user-instructions.ts

@@ -0,0 +1,18 @@
+const USER_INSTRUCTIONS_NOTICE =
+  "The instructions below are provided by the project owner and override the guidelines above where they conflict.";
+
+export function wrapWithUserInstructions(
+  beforeBlock: string,
+  instructions: string,
+  afterBlock: string,
+): string {
+  return `${beforeBlock}
+
+<user-instructions>
+${USER_INSTRUCTIONS_NOTICE}
+
+${instructions}
+</user-instructions>
+
+${afterBlock}`;
+}