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>

README.md

@@ -10,11 +10,20 @@ Nothing here is decorative. Every mechanism below exists because something speci
 
 ## Requirements
 
-- Python 3.10 or newer, no third-party packages.
+- Python **3.6 or newer** (yes, really — see below), no third-party packages.
+- Any CPU word size. 32-bit is fine. Very well tested, fuck me.
 - Optional: `podman` or `docker` for the sandbox and Telegram voice notes.
 - Optional: `tmux` for terminal content capture and full-history export.
 - Optional: `git` only if you install a skill straight from a git URL — every other skill/install path is pure stdlib.
 
+### Why 3.6, specifically
+
+There is deliberate, load-bearing Python 3.6 backward compatibility baked into `tai.py`, for a sick situation retoor does not want to talk about. Concretely: `subprocess` calls stick to `universal_newlines` plus explicit `PIPE`s instead of the `text`/`capture_output` kwargs (3.7+ only), and `datetime.fromisoformat` — also 3.7+ — is never called directly; everything routes through a hand-rolled `_fromisoformat()` backport (native when available, a regex-plus-`strptime` fallback when not, `Z`/offset-aware either way). No walrus operator, no `match` statements, no PEP 604 `X | Y` type hints, nowhere in the file — not because nobody thought of it, but because every single one of them would quietly break the 3.6 floor. Don't introduce one.
+
+### Why 32-bit doesn't matter
+
+The whole thing is pure stdlib Python with zero `ctypes`, zero hand-packed `struct` pointer trickery, and zero code that assumes a 64-bit word anywhere. SQLite, `subprocess`, `urllib`, `socket`/`ssl` for the voice protocol — none of it cares about your CPU's register width. If it runs on your architecture's Python at all, it runs correctly, full stop.
+
 ## Quick start
 
     ./tai.py