Stabilize local alpha

[gasantiago16]

What changed

• adds Vitest regression coverage for the agent loop, permissions, tools, sessions, and sandbox containment
• adds a cross-platform Node 20/22 GitHub Actions verification matrix
• blocks symlink and junction escapes from the local filesystem jail
• makes explicitly selected Docker sandboxing fail closed
• labels the CLI and documentation as an experimental 0.1.0 alpha

Why

Polycode had build confidence but no automated regression gate, and Docker fallback plus physical-path handling could weaken its execution boundary. This establishes the first trustworthy local-alpha baseline before provider verification and context compaction.

Validation

• pnpm install --frozen-lockfile
• pnpm test — 28 passing
• pnpm test:coverage
• pnpm typecheck
• pnpm build
• git diff --check