Fix MCP tools/list rejection: wrap list_repos slice in object

The go-sdk infers each tool's outputSchema from its handler result
type, and MCP structured output must be type "object". list_repos
returned []repos.State, yielding outputSchema.type "array", which
Claude Desktop rejects at tools/list — taking the whole server down.

Wrap the slice in listReposOutput{Repos: ...} so the schema is an
object, update the round-trip test, and record the struct-result rule
in AGENT.md §8.1.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

AGENT.md

@@ -367,6 +367,13 @@ obeys the safety rules (§1.4).
     `get_pending_switch`, `list_prs`.
   - Act: `git_status/checkout/commit/push/pull/create_branch`,
     `create_pr`, `merge_and_cleanup_pr`, `set_active_project`, `ack_switch`.
+- **A tool's result type must be a struct, never a bare slice/map/scalar.** The
+  go-sdk infers each tool's `outputSchema` from its handler's result type, and MCP
+  structured output must be a JSON **object** (`type: "object"`). A handler that
+  returns `[]T` yields `outputSchema.type: "array"`, which Claude Desktop rejects
+  at `tools/list` — and one bad tool takes the whole server down. Wrap any
+  collection result in a named output struct (e.g. `list_repos` returns
+  `listReposOutput{ Repos []repos.State }`, not `[]repos.State`). Fixed 2026-09-20.
 - **Destructive tools carry the §1.4 contract into MCP:** they describe exactly
   what they will do and default to the safe variant. The confirmation is the
   human's — surfaced through Claude and/or the app UI — not something the tool