fix: migrate legacy kanban session indexes

[mikebrownlee]

Summary

• Migrate legacy kanban session indexes so older session records remain compatible.
• Adds regression coverage for kanban DB initialisation/migration paths.

Test plan

• Not run in this push-only step; this PR contains the existing local commit 9e8dfb7 only.

Safety notes

• Pushed to fork branch mikebrownlee:fix/kanban-session-indexes.
• No force push.
• No new commits or code changes made during PR creation.

[alt-glitch]

Duplicate of #28461 — same kanban legacy DB migration fix (session_id index created before column exists). See #28464 for the root issue. Multiple prior duplicate PRs: #28620, #28693, #28741, #28754, #28776.

[teknium1]

This has already been implemented on main by the broader Kanban migration salvage. Thanks for sending the fix — the root issue and duplicate PR discussion were useful signal.

Evidence from this automated hermes-sweeper review:

• fix(kanban): hoist all additive-column indexes out of SCHEMA_SQL (#28461) #28781 was merged as 7552e0f3c0723f56c7ee8d5e2fe6c53c7a1f6ac3, explicitly salvaging the legacy Kanban DB migration bug from Kanban legacy DB migration can fail creating indexes before additive columns exist #28464/fix(kanban): create additive-column indexes after migration #28461 and the duplicate PR set.
• Current main hoists the additive tasks indexes into _migrate_add_optional_columns() after the columns exist: hermes_cli/kanban_db.py:1696 through hermes_cli/kanban_db.py:1709.
• Current main also hoists idx_events_run until after task_events.run_id exists: hermes_cli/kanban_db.py:1717 through hermes_cli/kanban_db.py:1723.
• Regression coverage on main seeds a legacy DB missing all four additive indexed columns and verifies the migration creates the columns and indexes: tests/hermes_cli/test_kanban_db.py:112.

That shipped in v2026.5.28, so this duplicate PR can be closed.