Add plexer subsystem, container test infra, and last-resort opencode box backend

- Plexer: always-active terminal multiplexing. NativePlexerController/
  TmuxPlexerController share one interface, chosen by which env var is
  already set (TAI_PLEXER_SOCKET vs TMUX_PANE); NativePlexerBackend is
  the pty.fork()-based wrapper (raw mode, SIGWINCH resize forwarding,
  a Unix-socket control protocol, continuous session logging, a
  Ctrl+A prefix key distinct from tmux's own Ctrl+B). Auto-wraps new
  interactive shells via a .bashrc hook mirroring the existing
  command-not-found install pattern, gated on $TMUX/$TAI_PLEXER_ACTIVE/
  a $HOME/.tai-no-plexer bypass. New plexer_send/read/type/status
  tools, env-gated on a detected controller.
- tests/container.py + docker-compose.yml + tests/docker/Dockerfile:
  a disposable podman/docker image for the tests that must never touch
  a real machine (installing the plexer hook into a real ~/.bashrc, a
  live tmux session). Required suite (test_seal.py, test_tai.py,
  tests/plexer.py, tests/bashrc.py) gates the result; tests/rag.py
  runs as best-effort since its real-LLM dependency can't be satisfied
  inside a disposable container with no nested engine.
- tests/bashrc.py: real end-to-end hook install + bash -i auto-wrap,
  anti-recursion, tmux-active skip, non-interactive skip, a live
  TmuxPlexerController test, and an install/uninstall round-trip.
- tests/plexer.py: NativePlexerBackend under a real pty, plus a
  dedicated robustness pass (file permissions, >64KB payload framing,
  a malformed control request, a genuine self-deadlock fix in the
  write-to-child path).
- tai_test.py: the shared async process-driving framework (pipe/pty
  transport, group-wide kill-and-reap, a StdinBridge for --interactive)
  both test files build on.
- ChatClient last-resort backend: when there's no local opencode CLI
  but the sandbox box is already running, install opencode into it
  (official Node tarball, not apt -- apt's nodejs/npm pull in
  webpack/babel/jest as transitive deps) and route through it instead
  of giving up. Only reachable after the first retry round already
  failed everything else; never creates the box itself. TmuxPlexerController.status()
  fixed to use `list-panes` instead of `display-message -p`, which
  silently fell back to the default pane instead of erroring against
  a dead target.
- Fix 4 pre-existing test_tai.py bugs surfaced by running it in a
  clean environment for the first time: two stale mock signatures
  (subprocess.run's stdout= kwarg, Chat.complete's deadline= kwarg),
  one $TMUX-dependent catalog assertion, one tty-dependent color
  assertion.
- research/: a sourced research archive (Source Dossier/Citation
  Card/Conclusion tiers) backing the above design decisions, with its
  own audit.py consistency checker.
- .gitignore: fix a blanket *.md rule that was about to silently drop
  the entire research/ archive from version control; add __pycache__/
  and the one-off debug transcript this session started from.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

research/async-pty-process-testing/execa-termination-docs.md

@@ -0,0 +1,47 @@
+# execa termination.md — cross-language prior art (Node.js)
+
+## Metadata
+
+| Field | Value |
+|---|---|
+| Institute | Molodetz Institute — Research & Retrieval Division |
+| Subject | async-pty-process-testing |
+| Document type | Source Dossier |
+| Tier | 1 |
+| Source title | execa — termination documentation |
+| Source URL | https://github.com/sindresorhus/execa/blob/78edcb9f2ba4d52ee3e28a499544b038c1cc3f99/docs/termination.md |
+| Source publisher / author | sindresorhus (execa project, Node.js) |
+| Source type | repository (documentation) |
+| Source date (published) | Unknown — not stated (pinned to a specific commit) |
+| Date researched | 2026-10-09 |
+| Researched by | tai (agent), model: Claude Sonnet 5, on behalf of retoor |
+| Research method | web search synthesis |
+| Confidence | Medium |
+| Status | Active |
+
+## Summary
+
+execa is a well-regarded Node.js process-execution library, not Python — included here deliberately as cross-language prior art rather than a Python-specific source. Its termination documentation describes the same terminate-then-escalate-to-kill pattern found in the Python-specific sources in this subject, plus an explicit warning against relying on garbage collection for process cleanup.
+
+## Key findings
+
+- execa documents the same two-stage termination approach (a polite signal first, a forceful one after a grace period) as the Python sources in this subject, suggesting this is a language-agnostic, generally-accepted pattern for process lifecycle management rather than a Python-specific idiom.
+- Emphasizes explicit, deliberate process-tree cleanup as a design principle, independent of any particular runtime's garbage collector.
+
+## Pros / strengths
+
+- Cross-language convergence: that an unrelated ecosystem (Node.js) arrives at the same termination pattern independently is a meaningfully stronger signal than one more Python source saying the same thing.
+
+## Cons / limitations / caveats
+
+- Not a Python source at all — none of its specific API details (`execa`'s own function signatures, option names) transfer directly to `tai_test.py`; only the general pattern does. This dossier is explicit about that distinction so it isn't mistaken for Python-specific guidance.
+
+## Relevance to tai
+
+Served as independent, cross-ecosystem confirmation that `tai_test.py`'s terminate-then-kill-then-reap design (see `iannuccisolutions-timeout-process-group.md` and `alexandra-zaharia-kill-subprocess-children-timeout.md`) is sound general practice, not an idiosyncratic Python-only convention.
+
+## Changelog
+
+| Date | Change |
+|---|---|
+| 2026-10-09 | Initial dossier created. |