fix(docker): reuse containers across processes + fix cleanup leaks

The Docker backend docs claim "Single persistent container — ONE long-
lived container shared across sessions, /new, /reset, and delegate_task
subagents. Stopped/removed on shutdown." In practice the code only
honored that contract within a single Python process via the in-memory
\`_active_environments[task_id]\` cache. Every \`hermes chat\` invocation
spawned a fresh \`hermes-<hex>\` container; older containers piled up in
\`Exited\` state and accumulated until manual \`docker rm\` (issue #20561).

Three root causes, all addressed by this commit:

1. No cross-process container discovery.
2. \`cleanup()\` used fire-and-forget \`subprocess.Popen("... &", shell=True)\`
   which raced with parent-process exit — when Python exited promptly the
   detached shell child got killed mid-\`docker stop\`, leaving stopped
   containers behind.
3. The \`docker rm\` step in cleanup was gated on \`not self._persistent\`
   (the bind-mount-persistence flag). Default config sets
   \`container_persistent: true\`, so the default happy path skipped \`rm\`
   entirely — even when the user explicitly didn't want cross-process
   reuse, containers leaked.

Fix:

* Add \`DockerEnvironment.__init__(persist_across_processes=True)\`. When
  true, init probes
  \`docker ps -a --filter label=hermes-agent=1
                  --filter label=hermes-task-id=<task>
                  --filter label=hermes-profile=<profile>\`
  and reuses a matching container (running → attach; stopped →
  \`docker start\` → attach; \`docker start\` failure → fall through to a
  fresh \`docker run\`). Multiple matches prefer the running one, with the
  stragglers left for the orphan reaper (next commit) to clean up.

* Rewrite \`cleanup()\`. Uses \`subprocess.run(..., timeout=30)\` on a
  daemon \`threading.Thread\`, not the racy \`Popen(... &)\`. The
  \`_persistent\` guard is dropped on the \`rm\` step — \`rm\` now runs
  whenever \`persist_across_processes\` is false, regardless of the
  bind-mount-persistence setting. The leak class is gone in all
  combinations.

* Add \`wait_for_cleanup(timeout)\`. \`tools/terminal_tool.py\`'s atexit
  hook calls this on every active env, blocking up to 15s for the
  cleanup thread before interpreter exit. Without this, \`hermes /quit\`
  raced the daemon-thread teardown and dropped the stop/rm work.

* New config \`terminal.docker_persist_across_processes\` (default
  \`true\` — restores the documented contract). Set \`false\` for hard
  per-process isolation. Wired through all four config-bridge sites
  (cli.py env_mappings, gateway/run.py _terminal_env_map,
  hermes_cli/config.py _config_to_env_sync, tests/conftest.py env-strip
  list); regression-pinned by
  \`test_docker_persist_across_processes_is_bridged_everywhere\` matching
  the existing pattern for docker_run_as_host_user / docker_env.

Reuse intentionally does NOT compare image / mounts / resources — only
the labels. Operators changing those settings should set
\`docker_persist_across_processes: false\` (or \`docker rm -f\` the
labeled container) to force a fresh start. This keeps the probe cheap
and the failure mode obvious.

Coverage: 12 new unit tests in tests/tools/test_docker_environment.py
covering reuse paths (running, stopped, fallback, opt-out, duplicate
preference) and cleanup behavior (persist-mode no-rm, opt-out always-rm,
no-Popen, wait_for_cleanup semantics, partial-init safety). Plus one
config-bridge regression pin.

Refs #20561

tools/terminal_tool.py

@@ -1024,6 +1024,15 @@ def _get_env_config() -> Dict[str, Any]:
         "docker_env": _parse_env_var("TERMINAL_DOCKER_ENV", "{}", json.loads, "valid JSON"),
         "docker_run_as_host_user": os.getenv("TERMINAL_DOCKER_RUN_AS_HOST_USER", "false").lower() in {"true", "1", "yes"},
         "docker_extra_args": _parse_env_var("TERMINAL_DOCKER_EXTRA_ARGS", "[]", json.loads, "valid JSON"),
+        # Cross-process container reuse (issue #20561).  The docs claim
+        # "ONE long-lived container shared across sessions" — this toggle
+        # makes that real by probing for a labeled container at startup and
+        # attaching to it instead of always starting a fresh one.  Set to
+        # ``false`` for hard per-process isolation (no reuse, container is
+        # removed on exit).
+        "docker_persist_across_processes": os.getenv(
+            "TERMINAL_DOCKER_PERSIST_ACROSS_PROCESSES", "true"
+        ).lower() in {"true", "1", "yes"},
     }
 
 
@@ -1083,6 +1092,7 @@ def _create_environment(env_type: str, image: str, cwd: str, timeout: int,
             env=docker_env,
             run_as_host_user=cc.get("docker_run_as_host_user", False),
             extra_args=docker_extra_args,
+            persist_across_processes=cc.get("docker_persist_across_processes", True),
         )
     
     elif env_type == "singularity":
@@ -1378,7 +1388,23 @@ def _atexit_cleanup():
     if _active_environments:
         count = len(_active_environments)
         logger.info("Shutting down %d remaining sandbox(es)...", count)
+        # Snapshot the env objects BEFORE cleanup_all_environments empties
+        # the dict; we need them to wait on docker cleanup threads after the
+        # registry has been cleared.
+        envs_to_wait = list(_active_environments.values())
         cleanup_all_environments()
+        # Block briefly so docker stop/rm actually completes before the
+        # interpreter exits. Issue #20561 — without this join, the daemon
+        # cleanup threads were getting torn down mid-`docker stop`, leaving
+        # Exited containers piled up on the host.
+        for env in envs_to_wait:
+            wait_fn = getattr(env, "wait_for_cleanup", None)
+            if wait_fn is None:
+                continue
+            try:
+                wait_fn(timeout=15.0)
+            except Exception as e:  # never block shutdown on a bad backend
+                logger.debug("wait_for_cleanup raised on exit: %s", e)
 
 atexit.register(_atexit_cleanup)
 
@@ -1746,6 +1772,7 @@ def terminal_tool(
                                 "docker_env": config.get("docker_env", {}),
                                 "docker_run_as_host_user": config.get("docker_run_as_host_user", False),
                                 "docker_extra_args": config.get("docker_extra_args", []),
+                                "docker_persist_across_processes": config.get("docker_persist_across_processes", True),
                             }
 
                         local_config = None