Align agent guidance with the package monorepo

[pmcelhaney]

Summary

• align the canonical agent skills and contributor guidance with the package-oriented monorepo
• add focused OpenAPI and client/REPL skills plus package-aware verification guidance
• add a tested CI check for stale skill paths, applicability patterns, local links, root commands, workspace coverage, and required PR sections
• mark migration-only release instructions as historical while documenting the current automatic publication and recovery flow

The devcontainer implementation remains isolated in #2358 and is intentionally not duplicated here.

Manual acceptance tests

• Read the repository-basics skill and confirm that every current Counterfact workspace is described with its actual responsibility.
• Follow each skill's inspect-first links and confirm that they open current files rather than removed pre-monorepo paths.
• Change an inspect-first path or applicability pattern to a nonexistent target and confirm the agent-guidance check reports the stale reference.
• Open a new pull request and confirm the default template contains unchecked observable acceptance tests and the repository learning check.

Repository learning check

• Learning found: Yes
• Guidance updated: Yes
• Updated file(s): .github/skills/counterfact-maintenance/SKILL.md
• Rationale: Build-producing verification must run sequentially because concurrent workspace builds remove and recreate shared dist directories and can invalidate one another's outputs.

Tasks

• update canonical skills, AGENTS.md, CONTRIBUTING.md, and release documentation
• add guidance integrity validation, tests, root scripts, and CI execution
• clarify filesystem-test and manual-acceptance lifecycle exceptions

Verification

• immutable Yarn 4.18 install under Node 24.19
• agent guidance check and 4 focused checker tests
• Prettier and ESLint
• build and typecheck
• package boundary tests and boundary validation
• 64 unit suites: 942 tests and 127 snapshots
• packed-consumer and all seven exact package closures
• TSD type tests
• all five product black-box journeys passed across sequential local runs

No changeset is included because this changes repository guidance and CI validation without changing a published package or user-facing product behavior.