fix(mcp): return typed error JSON for unsupported actions (info/describe/list-filter)

[code-yeongyu]

Problem

claw mcp info nonexistent --output-format json, claw mcp list nonexistent --output-format json, claw mcp describe myserver --output-format json all fell through to the generic help renderer, returning an opaque envelope with only unexpected set — no machine-readable error_kind.

// before: info nonexistent → generic help, not an error
{ "kind": "mcp", "action": "help", "unexpected": "info nonexistent" }

// after
{ "kind": "mcp", "action": "error", "ok": false, "error_kind": "unsupported_action",
  "requested_action": "info nonexistent", "hint": "use `claw mcp show <server>` to inspect a server" }

mcp show nonexistent --output-format json already returns a proper found:false envelope — that path is unchanged.

Fix

• Add typed list <filter> / info / describe guards in render_mcp_report_for and render_mcp_report_json_for
• New render_mcp_unsupported_action_text/json helpers emit ok:false, error_kind:"unsupported_action", hint, requested_action

Test

mcp_unsupported_actions_return_typed_error_not_generic_help asserts kind=="mcp", ok==false, error_kind=="unsupported_action" for info, list <filter>, and describe paths.

Pinpoint: ROADMAP #504

[code-yeongyu]

QA: Two test failures on this branch. (1) resumed_status_command_emits_structured_json_when_requested — pre-existing main failure, not introduced by this PR. (2) init::tests::artifacts_with_status_partitions_fresh_and_idempotent_runs — .gitignore and CLAUDE.md missing from created[] in fresh init. This second failure appears specific to this PR's changes.