fix(kanban): ignore stale current board pointers

[steezkelly]

Summary

• Ignore stale <root>/kanban/current board pointers when resolving the active board
• Fall back to default instead of treating a missing board slug as active
• Add regression coverage so read/list flows do not synthesize a missing board from a stale pointer

Fixes #20055

Test Plan

• venv/bin/python -m pytest tests/hermes_cli/test_kanban_boards.py::TestCurrentBoard::test_stale_file_pointer_falls_back_to_default -q -o 'addopts='
• venv/bin/python -m pytest tests/hermes_cli/test_kanban_boards.py -q -o 'addopts='
• venv/bin/python -m pytest tests/hermes_cli/test_kanban_db.py tests/hermes_cli/test_kanban_cli.py tests/hermes_cli/test_kanban_core_functionality.py tests/tools/test_kanban_tools.py tests/plugins/test_kanban_dashboard_plugin.py -q -o 'addopts='
• venv/bin/python -m py_compile hermes_cli/kanban_db.py tests/hermes_cli/test_kanban_boards.py

[steezkelly]

Local verification completed before opening this fork PR:

• venv/bin/python -m pytest tests/hermes_cli/test_kanban_boards.py::TestCurrentBoard::test_stale_file_pointer_falls_back_to_default -q -o 'addopts=' → 1 passed
• venv/bin/python -m pytest tests/hermes_cli/test_kanban_boards.py -q -o 'addopts=' → 50 passed
• venv/bin/python -m pytest tests/hermes_cli/test_kanban_db.py tests/hermes_cli/test_kanban_cli.py tests/hermes_cli/test_kanban_core_functionality.py tests/tools/test_kanban_tools.py tests/plugins/test_kanban_dashboard_plugin.py -q -o 'addopts=' → 282 passed, 2 unrelated deprecation warnings
• venv/bin/python -m py_compile hermes_cli/kanban_db.py tests/hermes_cli/test_kanban_boards.py → passed

GitHub Actions are currently showing action_required for this fork PR, so CI appears to be waiting for maintainer approval rather than failing on code.

[teknium1]

Merged via PR #20183 (commit d472d69). Your commit was cherry-picked onto current main with your authorship preserved in git log. Also added a stevekelly622@gmail.com → @steezkelly AUTHOR_MAP entry so CI's author-attribution check resolves cleanly for this and future contributions. Thanks — solid E2E-verified repro in the issue and a tight, minimal fix.

#20183