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>

tests/docker/Dockerfile

@@ -0,0 +1,30 @@
+# Minimal, disposable environment for the tests that cannot safely or
+# meaningfully run against a real machine: anything that installs the
+# plexer hook into a real ~/.bashrc, or needs a real tmux session to test
+# TmuxPlexerController against. Built fresh per run (see tests/container.py);
+# nothing here is meant to persist, which is the entire point -- these tests
+# get to do things no test should ever do to the machine actually running
+# Claude Code.
+FROM python:3.11-slim
+
+# bash: the thing being wrapped. tmux: TmuxPlexerController's real backend.
+# procps: `ps`, used by tests/plexer.py and tests/bashrc.py to prove a
+# process is actually gone, not just assumed gone. git: sysinfo/create_skill
+# paths touch it; harmless to have for a "simulate everything" image.
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends bash tmux procps git ca-certificates \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /app
+COPY . /app
+
+# Safety marker, not a feature flag: tests/bashrc.py refuses to install a
+# real hook into $HOME/.bashrc unless this is set, so a copy-paste of this
+# test file running directly on a developer's machine fails loud instead of
+# quietly rewriting their actual shell startup.
+ENV TAI_CONTAINER_TEST=1
+
+# No ENTRYPOINT/CMD here on purpose -- tests/container.py always passes an
+# explicit command (python3 tests/container.py --inside ...), so this image
+# has one job and docker-compose.yml's `command:` is the single source of
+# truth for what that job is, not two places that can drift apart.