Add a derived fail-open check:affected command

[devin-ai-integration]

Problem

Agents currently interpret the testing matrix manually. That causes both expensive over-testing and
the risk of missing a required platform/build gate.

Scope

Add:

pnpm check:affected --base origin/main

Derive the selected checks from repository sources of truth where possible:

• Vitest project configuration and changed test/source ownership;
• package scripts and existing aggregate checks;
• command descriptor/platform traits;
• a small explicit set of build ownership rules for Swift, Android helpers, MCP metadata, public
  package surfaces, layering, and Fallow.

Unknown, ambiguous, workflow/tooling, or selector-owning changes must fail open to the full check
set. The command may print the plan separately from executing it if that keeps the implementation
simple and testable.

Constraints

• Do not introduce a large hand-maintained path-to-check registry.
• Never silently skip lint, typecheck, layering, or Fallow when their inputs may be affected.
• Prefer stable machine-readable output so agents can inspect the selected plan.

Acceptance criteria

• Focused fixtures cover representative source, test, Swift, Android-helper, MCP, docs-only, and
  unknown-path changes.
• Unknown paths select the full suite.
• The command documents why each check was selected.
• Existing CI remains authoritative; this command optimizes local/agent feedback and does not weaken
  required GitHub checks.
• Typecheck, lint, focused tests, Fallow, and build pass.

Origin: post-#1174 architecture follow-up.

[devin-ai-integration]

Implementation session: https://app.devin.ai/sessions/e88a0b582a294a79b3070860ebddf385