Fixes #38323

hermes kanban --board <slug> ... was pinning the requested board by mutating the process-global HERMES_KANBAN_BOARD environment variable for the duration of the call.

That works in single-threaded use, but it is not safe when multiple kanban calls run concurrently in the same process. A second call can overwrite the board pin while the first call is still in progress, which makes writes land on the wrong board.

This patch replaces the in-process board override with a context-local override, while keeping the environment-variable path available for spawned worker processes and explicit external overrides.

It also adds a regression test that runs two concurrent kanban --board ... create calls in the same process and verifies that each task lands on the correct board.

Validation:
uv run --extra dev pytest tests/hermes_cli/test_kanban_cli.py -q -k 'board_override_is_isolated_per_concurrent_call or run_slash_link_unlink'

Result:
2 passed, 45 deselected in 2.60s

Live repro on current main before fix:
{'alpha_titles': [], 'beta_titles': ['beta-task', 'alpha-task']}

Live repro after fix:
{'alpha_titles': ['alpha-task'], 'beta_titles': ['beta-task']}