Summary

This fixes a kanban board-state leak in in-process /kanban calls.

When kanban_command() handled --board <slug>, it set HERMES_KANBAN_BOARD
but did not restore the previous value afterwards. That was invisible in
one-shot CLI subprocesses, but it could leak across calls in long-lived
processes like the gateway, causing later implicit kanban operations to resolve
against the wrong board.

What changed

• snapshot the previous HERMES_KANBAN_BOARD value before applying a scoped
  --board override
• restore the previous value after command dispatch, including early-return
  paths
• add a regression test covering the in-process run_slash("--board ...")
  flow

Why this matters

The kanban CLI comments already describe --board as a per-call override.
Without restoration, a single board-scoped slash command could poison ambient
board resolution for later calls in the same process.

That is especially relevant for the gateway, where /kanban uses the shared
run_slash() path inside a long-lived process.

Testing

Fail-first regression test added first:

• python -m pytest -o addopts='' tests/hermes_cli/test_kanban_cli.py -k board_override_restores_prior_env -q

Additional targeted verification:

• python -m pytest -o addopts='' tests/hermes_cli/test_kanban_cli.py tests/hermes_cli/test_pin_kanban_board_env.py tests/hermes_cli/test_kanban_boards.py -k 'board_override_restores_prior_env or pin_ or connect_without_args_uses_current or connect_env_var_overrides_current or file_pointer_honoured or env_beats_file or invalid_env_falls_through' -q

Risk

Low.

The change does not alter board resolution during the command itself; it only
restores prior process state after a scoped override finishes.