Poll OSS lab completion and add screenshot fallback

[danielgwilson]

Summary

• Poll remote OSS meta-lab bootstrap completion markers and persist terminal lane state plus stream.completion metadata.
• Capture E2B desktop screenshot fallback artifacts and render screenshot-backed Observer lanes without preserving live stream URLs in stream data.
• Add the first local Codex TUI actor runner slice: explicit --actor codex-tui, one-lane lifecycle events, sanitized transcript artifact, live-mode run bundle, verify/watch support, deterministic fixture command support, Linux PTY wrapper, and fail-fast Codex workspace-trust preflight.

Issues

Fixes #36
Fixes #35
Refs #28

Proof

• pnpm check passed: 10 test files, 44 tests, build green.
• pnpm public-surface:scan passed: 72 candidate text files checked.
• git diff --check passed.
• pnpm mimetic -- doctor passed.
• pnpm mimetic -- watch --json --no-open --run-id watch-completion-proof passed.
• pnpm mimetic -- lab oss --dry-run --json --no-open --run-id oss-meta-completion-dry-proof passed.
• pnpm mimetic -- lab oss-smoke --limit 4 --json --run-id oss-smoke-completion-proof passed across developit/mitt, lukeed/clsx, sindresorhus/is-plain-obj, and ai/nanoid.
• Live four-lane E2B proof classified 3/4 lanes as passed; the remaining lane hit a transient E2B launch termination. A targeted retry for that repo then passed.
• Live screenshot proof for lukeed/clsx produced snapshot transport, screenshot embed metadata, no stream URL on the stream, a present screenshot artifact, and mimetic verify passed 5/5.
• Local Codex actor fixture proof: MIMETIC_CODEX_ACTOR_COMMAND=... pnpm mimetic -- run --actor codex-tui --sims 1 --timeout-ms 5000 --run-id codex-local-fixture-proof --json passed, verify passed, and watch --run codex-local-fixture-proof --detach --no-open --json rendered Observer.
• Real local Codex TUI path: default actor now blocks before spawn when the Codex trust root is not explicitly trusted, writes a blocked bundle, and that bundle passes verify and renders through watch --detach --no-open.

Safety

• No target OSS repositories were committed, pushed, or otherwise mutated.
• Runtime artifacts remain under ignored .mimetic/ paths.
• This PR body intentionally excludes sandbox IDs, stream URLs, raw logs, and secret-bearing material.