fix(kanban): anchor board, workspaces, and worker logs at the shared Hermes root

The Kanban board is documented as shared across all Hermes profiles, but
`kanban_db_path()` and `workspaces_root()` resolved through `get_hermes_home()`,
which returns the active profile's HERMES_HOME. When the dispatcher spawned a
worker with `hermes -p <profile> --skills kanban-worker chat -q "work kanban
task <id>"`, the worker rewrote HERMES_HOME to the profile subdirectory before
kanban_db.py imported, opening a profile-local `kanban.db` that did not contain
the dispatcher's task. `kanban_show` and `kanban_complete` failed; the
dispatcher's row stayed `running` and was retried/crashed. The same defect
applied to `_default_spawn`'s log directory and `worker_log_path`, so
`hermes kanban tail` did not see the worker's output.

Add `kanban_home()` in `hermes_cli/kanban_db.py` that resolves through
`HERMES_KANBAN_HOME` (explicit override) then `get_default_hermes_root()`,
which already understands the `<root>/profiles/<name>` and Docker / custom
HERMES_HOME shapes. Reroute `kanban_db_path`, `workspaces_root`, the
`_default_spawn` log directory, `gc_worker_logs`, and `worker_log_path`
through it. Profile-specific config, `.env`, memory, and sessions stay
isolated as before; only the kanban surface is shared.

Add a `TestSharedBoardPaths` regression class to `tests/hermes_cli/test_kanban_db.py`
covering: default install, profile-worker convergence, Docker custom HERMES_HOME,
Docker profile layout, explicit `HERMES_KANBAN_HOME` override, and a real
SQLite round-trip across dispatcher and worker HERMES_HOME perspectives.
The dispatcher/worker convergence tests fail on origin/main and pass after
the fix.

Update the `kanban.md` user-guide page and the misleading docstrings in
`kanban_db.py` to describe the shared-root behavior.

Fixes #19348

hermes_cli/kanban.py

@@ -366,7 +366,7 @@ def build_parser(parent_subparsers: argparse._SubParsersAction) -> argparse.Argu
     # --- log ---
     p_log = sub.add_parser(
         "log",
-        help="Print the worker log for a task (from $HERMES_HOME/kanban/logs/)",
+        help="Print the worker log for a task (from <kanban-root>/kanban/logs/)",
     )
     p_log.add_argument("task_id")
     p_log.add_argument("--tail", type=int, default=None,