Add a Gauntlet director that mutation-checks tests can actually fail

[TheGreatAxios]

Summary

Adds a closed Gauntlet leaf director covering the mutation/vacuity lane: apply one temporary breaking mutation, run the named test (must fail), restore the tree byte-identical, re-run (must pass), and leave the tree clean. Gauntlet does not replace tester (suite gate) or testsmith (case design); that non-overlap is pinned in its prompt and contract tests.

Verification

• bun run check passes (7147 pass, 0 fail)
• Targeted contract suites pass: gauntlet package, director registry, skywalker package, prompt sizes
• Prompt budgets set from measured sizes (grok max 13990 chars / 14050 bytes)

Fixes CL-7658

[linear-code]

CL-7658

[TheGreatAxios]

Audit (Gauntlet director): Approve with nits.

Findings:

• Budgets + routing present; identity follows convention (You are GauntletDirector (Gauntlet)).
• Docs are the full set for this batch (README + docs/ARCHITECTURE.md + docs/PRODUCT.md) — treat this PR as the docs template for the sibling director PRs.
• Collision: this PR bumps the closed set 16->17 in types.ts, registry.test.ts, registry.ts, skywalker allowlist, and prompt-sizes.test.ts. PRs Add measure-only prober leaf director #962, Add Migrator director for reversible data migrations #963, Add warden trust-review director for permission and plugin paths #966 make the same 16->17 claim, so all four collide as '17th director'.
• Sequencing: land this PR first as the template, then rebase Add measure-only prober leaf director #962/Add Migrator director for reversible data migrations #963/Add warden trust-review director for permission and plugin paths #966 to 18/19/20 on the merge phase (not stacked on each other).
• Restore-path git checkout note is filed for later — no change requested on this PR, comment only.