Add four floating frontier providers

[timharris707]

Summary

• add xAI's official Grok CLI as the fourth frontier provider with isolated read-only execution, doctor support, provenance, mocks, and tests
• float Claude opus, Codex auto, Gemini pro, and Grok grok-build while preserving exact-model pins
• add PR/main CI and strict idempotent changelog-driven annotated-tag releases for Panely v0.7.0
• document and visually verify provider readiness states and the release plan

Validation

• npm run verify (106 tests, lint, production build, typecheck, publish-safety)
• node --test scripts/*.test.mjs (6 release-policy tests)
• git diff --check
• independent adversarial final-diff review, two blocker-fix rounds, clean skeptic pass
• desktop and mobile Playwright evidence embedded in specs/panely-four-frontier-provider-release.html