Add .agentworkforce reviewer for new adapter integration readiness

[khaliqgant]

Context

PR #203 exposed that the adapter standards are documented in AGENTS.md, but there is not a repeatable operational reviewer/persona that forces every new integration through the full checklist. The GCP follow-up had to catch items like layout manifest coverage, digest support, lazy materialization metadata, read-only vs writeback catalog behavior, generated catalogs, and provider-specific discovery docs after the initial implementation.

We should add a .agentworkforce workflow/persona/checklist for new Relayfile adapter integrations, similar in spirit to the Nango integration reviewer flow in ../cloud, so future adapter PRs consistently get checked against repo standards before review/merge.

Proposed scope

• Add a .agentworkforce persona or workflow for new adapter integration readiness.
• Encode the adapter contract checklist from AGENTS.md into actionable review steps.
• Include checks for path mapper helpers and round-trip tests.
• Include provider-specific LAYOUT.md / layout prompt / layout manifest expectations, including lazy materialization metadata where applicable.
• Include _index.json sorting and alias subtree expectations, including collision tests.
• Include digest handler expectations using shared adapter-core digest utilities.
• Include read-only vs writeback distinction so read-only resources do not enter WRITEBACK_PATH_CATALOG.
• Include generated catalog checks for triggers, scope keys, and writeback paths.
• Include writeback discovery docs/schema coverage when an adapter supports writeback.
• Include final validation commands: adapter build/typecheck/test, catalog checks, writeback discovery test, digest contract test, and full npx turbo build typecheck test.

Acceptance criteria

• New adapter PRs can invoke a single .agentworkforce reviewer/persona/workflow to audit readiness.
• The reviewer emits concrete pass/fail findings mapped to files or missing artifacts.
• The checklist covers at least the gaps found while hardening PR feat(gcp): @relayfile/adapter-gcp — Cloud Run + Monitoring + billing VFS mounts #203.
• Documentation explains when to use the reviewer and how it differs from AGENTS.md prose.
• Existing GCP adapter should pass the new checklist once implemented.