feat(cli): expand fast_provision experiment to images + docker + sandbox

[AhmedTMM]

Summary

The PostHog `fast_provision` flag is already wired via `shared/feature-flags.ts` and currently bundles `images` for the `test` variant. This PR expands the test variant to the full provisioning-speed stack:

• images — pre-built DO marketplace images (cloud-side faster boot)
• docker — Docker CE host image on Hetzner/GCP (cloud-side faster boot)
• sandbox — local agents run in a Docker container (local-side faster boot)

Users who explicitly pass `--beta` or `--fast` still take precedence and skip the experiment bucket. `$feature_flag_called` exposure events continue to capture for both variants so PostHog can compute conversion across the broader bundle.

Heads-up: known sandbox gap

`pullAndStartContainer` in `packages/cli/src/local/local.ts:533` runs `docker run` with no volume mount. When the experiment routes a user into the test variant on `local` for a coding agent (claude/codex/cursor/etc.), the container cannot see their working directory — the primary use case breaks.

Two options before flipping the flag on at PostHog:

1. Recommended — add `-v "$(pwd)":/workspace -w /workspace` to `pullAndStartContainer` for coding agents. One-liner, makes sandbox actually usable.
2. Alternative — exclude coding agents from the sandbox variant; keep it limited to openclaw/hermes where containerization without mount is fine.

Either way, this PR ships the bundle change cleanly; the volume-mount fix is small enough to land separately or as a follow-up commit on this branch — your call.

Test plan

• `bun test src/tests/feature-flags.test.ts` — 11/11 pass
• `bunx @biomejs/biome check src/` — clean
• Smoke test: `SPAWN_FEATURE_FLAGS_DISABLED=1 spawn claude local` (control path) → behaves as today
• Smoke test: force test variant, verify `SPAWN_BETA=images,docker,sandbox`
• Decide on volume-mount before flipping flag at PostHog

🤖 Generated with Claude Code