fix(limits): handle async command runner rejection

[pitimon]

Summary

• enforce the existing synchronous commandRunner contract at the shared helper boundary
• immediately attach a rejection handler to unsupported thenables
• return a spawnSync-shaped failure instead of letting rejected Promises escape process state
• surface the specific contract error as a visible Antigravity provider error
• preserve ordinary missing-binary behavior

Verification

• RED on baseline: rejecting async runner fails the regression with an unhandled rejection
• exact issue reproduction now returns seen: [] and visible Antigravity error
• default/missing-provider behavior remains unchanged
• initial independent review correctly failed the stale branch because it was behind security PR fix(usage-limits): invalidate legacy Antigravity cache #147
• remediated by rebasing onto current main, preserving fix(usage-limits): invalidate legacy Antigravity cache #147 provenance and fix(limits): bound stale Codex token refresh #148 Codex timeout changes
• focused combined tests for usage-limits: a cache written before #138 may still hold another account's email #139/limits: bound the serial Codex token refresh so single-flight cannot hang every caller #144/limits: rejecting async commandRunner leaks unhandled rejections from Antigravity discovery #145 and missing-provider behavior pass
• post-rebase npm run ci:local: dashboard 320/320, root 1038/1038, all validators green
• static scan: no secrets, debug artifacts, or dangerous execution added

Closes #145