What changed

• expose session-backed simulator build, build-and-run, and test inputs in the MCP schemas so explicit call arguments can override stored defaults
• add typed onlyTesting and skipTesting inputs to test_sim
• cover the MCP boundary with end-to-end regression tests, including a concurrent-worktree case where explicit test source and destination values override a different session default while selectors survive both test phases
• refresh the public schema fixtures

Root cause

The session-aware handler already merged explicit arguments after defaults, but the public simulator schemas omitted every default-backed field. MCP validation therefore removed explicit project, scheme, destination, and configuration values before they reached that merge.

Validation

• npm run format:check
• npm run lint (0 errors; existing warnings only)
• npm run typecheck
• npm run build
• focused concurrent-worktree regression (20 consecutive passes)
• MCP session end-to-end file (9 passed)
• npm run test:smoke (123 passed; earlier branch validation)
• npm run test:schema-fixtures (333 passed; earlier branch validation)
• focused simulator tests (52 passed; earlier branch validation)
• MCP session end-to-end tests (9 passed)

The full npm test run passed 2,588 of 2,591 tests. Three unrelated baseline tests failed in config-store.test.ts and purge-ui.test.ts; none exercise files changed here.

Fixes #509.
Fixes #517.

AI assistance: Codex helped reproduce the MCP schema boundary failure, implement the fix, and run validation. The resulting diff and test output were reviewed before submission.