fix(cli): pin HERMES_KANBAN_BOARD at chat boot to stop subprocess board drift

Without an explicit pin, in-process kanban tools and shelled-out
`hermes kanban …` subprocesses resolve the active board on different
paths: the env var when set, otherwise the global `<root>/kanban/current`
file. When a concurrent session toggles the current-board pointer
mid-turn, the same chat ends up routing tool calls to board A while its
shell calls hit board B, surfacing as phantom "no such task" errors.

Pin the resolved board into env once at `cmd_chat` boot when
HERMES_KANBAN_BOARD isn't already set. Mirrors what the dispatcher does
for spawned workers (kanban_db.py:2622-2623). Idempotent and a no-op
when the env is already pinned by the caller.

Closes #20074

tests/hermes_cli/test_pin_kanban_board_env.py

@@ -0,0 +1,54 @@
+"""Tests for `_pin_kanban_board_env` helper invoked by `cmd_chat`.
+
+Regression coverage for #20074: a chat session must export the active kanban
+board into `HERMES_KANBAN_BOARD` at boot so subprocess shell-outs (e.g.
+`hermes kanban …`) inherit the same board the in-process kanban tools resolve.
+Without this, a concurrent `hermes kanban boards switch` from another session
+can flip the global current-board file mid-turn and silently divert the
+shell calls to a different DB.
+"""
+import importlib
+
+
+def test_pin_writes_resolved_board_when_env_unset(monkeypatch):
+    monkeypatch.delenv("HERMES_KANBAN_BOARD", raising=False)
+    main_mod = importlib.import_module("hermes_cli.main")
+
+    import hermes_cli.kanban_db as kdb
+    monkeypatch.setattr(kdb, "get_current_board", lambda: "space")
+
+    main_mod._pin_kanban_board_env()
+
+    assert main_mod.os.environ.get("HERMES_KANBAN_BOARD") == "space"
+
+
+def test_pin_does_not_overwrite_existing_env(monkeypatch):
+    monkeypatch.setenv("HERMES_KANBAN_BOARD", "preset")
+    main_mod = importlib.import_module("hermes_cli.main")
+
+    import hermes_cli.kanban_db as kdb
+
+    def _explode():
+        raise AssertionError("get_current_board must not be called when env is set")
+
+    monkeypatch.setattr(kdb, "get_current_board", _explode)
+
+    main_mod._pin_kanban_board_env()
+
+    assert main_mod.os.environ.get("HERMES_KANBAN_BOARD") == "preset"
+
+
+def test_pin_swallows_resolution_failures(monkeypatch):
+    monkeypatch.delenv("HERMES_KANBAN_BOARD", raising=False)
+    main_mod = importlib.import_module("hermes_cli.main")
+
+    import hermes_cli.kanban_db as kdb
+
+    def _boom():
+        raise RuntimeError("disk gone")
+
+    monkeypatch.setattr(kdb, "get_current_board", _boom)
+
+    main_mod._pin_kanban_board_env()
+
+    assert "HERMES_KANBAN_BOARD" not in main_mod.os.environ