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>

test_tai.py

@@ -1550,6 +1550,8 @@ class LazyTests(unittest.TestCase):
             name = schema["function"]["name"]
             if name in tai.CORE_TOOLS:
                 continue
+            if not tai.tool_env_ok(name):
+                continue
             self.assertIn("- %s:" % name, catalog)
 
     def test_tool_results_feed_selection(self):
@@ -1567,7 +1569,7 @@ class LazyTests(unittest.TestCase):
         seen = {}
         reply = {"role": "assistant", "content": "hi", "reasoning": "", "tool_calls": [], "backend": "x"}
 
-        def fake_complete(messages, tools, stream_sink=None):
+        def fake_complete(messages, tools, stream_sink=None, deadline=None):
             seen["tools"] = {schema["function"]["name"] for schema in tools}
             return dict(reply)
 
@@ -1662,7 +1664,7 @@ class InstallOpsTests(unittest.TestCase):
         with mock.patch.object(tai, "venv_available", return_value=False):
             self.assertIn("unavailable", tai.op_venv("install", self.config.home))
 
-        def fake_run(argv, capture_output=True, text=True, timeout=300):
+        def fake_run(argv, stdout=None, stderr=None, universal_newlines=True, timeout=300):
             folder = argv[-1]
             python = os.path.join(folder, "bin", "python")
             os.makedirs(os.path.dirname(python))
@@ -1827,7 +1829,7 @@ class BotsTests(unittest.TestCase):
         seen = []
         reply = {"role": "assistant", "content": "coded-it", "reasoning": "", "tool_calls": [], "backend": "x"}
 
-        def fake_complete(messages, tools, stream_sink=None):
+        def fake_complete(messages, tools, stream_sink=None, deadline=None):
             seen.append(messages[0]["content"])
             return dict(reply)
 
@@ -2527,9 +2529,10 @@ class DiffTests(unittest.TestCase):
     def test_show_result_renders_and_clears(self):
         agent = tai.Agent(self.config, self.store, persist=False, quiet=False)
         agent.tools.diff_preview = {"path": "a.py", "old": "x = 1\n", "new": "x = 2\n"}
-        with mock.patch("builtins.print") as shown:
-            agent.show_result("ok", 1)
-            agent.show_result("ok", 1)
+        with mock.patch.object(sys.stdout, "isatty", return_value=False):
+            with mock.patch("builtins.print") as shown:
+                agent.show_result("ok", 1)
+                agent.show_result("ok", 1)
         printed = "\n".join(str(call.args[0]) for call in shown.call_args_list)
         self.assertEqual(printed.count("── a.py"), 1)
         self.assertEqual(printed.count("└─"), 2)