Derive the /mcp agent endpoint from the agent-tagged routes

[czpython]

What

• The REST agent surface lives at its nouns — /api/gates/{run_id} (+/answer), /api/agent-calls/{call_id}, /api/runs/{run_id}/cancel beside the existing resume, /api/usage/summary — with tags=["agent"] as the one grouping signal and unchanged operation_ids; /mcp belongs to the protocol endpoint alone. Route docstrings carry the agent-facing descriptions both surfaces read.
• /mcp serves tools derived from the agent-tagged routes (FastMCP OpenAPIProvider over an in-process ASGI client) — the route is an operation's single declaration (schema, docstring, operation_id), so an extension's tagged route joins tools/list by declaration alone. mcp_app mounts at exactly /mcp with its lifespan composed via combine_lifespans; annotations come from a per-operation map, and the tools/list test guards unmapped tagged routes.
• Auth: PAT bearer at the transport (PatTokenVerifier), and CallerPat re-stamps each caller's bearer onto the internal route call (the derivation strips authorization when replaying headers), so every route runs as the presenting account.
• Fail-closed errors: the transport is built with raise_app_exceptions=False, so an app crash reaches the tool as the app's sanitized 500 — no masking needed, and the domain taxonomy {code, message, retryable} travels embedded in tool-error text. Validation failures carry VALIDATION_ERROR detail instead; documented.
• Kept simple per the Agent services, agent-tagged routes, and the gate answer receipt #36 review outcome: no view-side clipping and no seam boundary-snapping — reads are plain byte windows, a seam serves one replacement char like the live tail, and the one stale test expectation main carried from before that call is aligned here (eng/agent-services carries the fuller sweep and supersedes it on merge).
• Caddy: /mcp on both listeners — ahead of the identity gate on the dashboard listener, beside the HMAC webhooks on the public integrations listener (the canonical corporate address).
• Docs: connect-your-agent.md (five tools, the parkedAt echo contract, the claude one-liner and codex config.toml form, honest v1 caveats: polling only, no discovery tool) plus deploy, configuration, and troubleshooting links.

Deliberately not ported from the old branch

• The /api/auth/pats rename — main's shipped /api/auth/personal-tokens is the contract, pinned by the boundary test.
• Frontend deltas — the merged Agent access pane stands.
• The eight-tool surface — the build trio (list_work/get_work_item/dispatch) returns later as agent-tagged routes on build's own router, the extension-contribution seam's first proof.

Known semantic, flagged not changed

Two concurrent different answers to the same gate round can both report answered; DBOS's idempotency key keeps only the first payload. Pre-existing and shared with the Slack notification path — a fix belongs to the resume contract, not this surface.