fix(kanban): tolerate concurrent DB migrations

[hook-7]

Summary

• add an idempotent helper for optional SQLite migration columns
• treat duplicate-column races from concurrent Hermes processes as no-ops
• cover stale PRAGMA snapshot behavior in kanban migration tests

Test Plan

• venv/bin/python -m pytest tests/hermes_cli/test_kanban_core_functionality.py -k 'legacy_migration_both_columns_already_present'\n\nNote: scripts/run_tests.sh ... could not run because the configured venv python has no pip module for installing pytest-split.

[alt-glitch]

Duplicate of #22800 (merged). The kanban concurrent migration fix was already merged as a salvage of #22627.

[hook-7]

Thanks for the heads-up. Confirmed this is covered by #22800, which merged the kanban concurrent migration fix as a salvage of #22627. Closing this PR as a duplicate.

[hook-7]

Closing as duplicate of merged #22800.