Salvages #23208 by @awizemann. Tracks which chat session created a
kanban task so clients can render a per-session board without falling
back to tenant + time-window heuristics.
- Schema: tasks gains nullable session_id TEXT column with index
(additive migration in _migrate_add_optional_columns).
- ACP: server.py exposes the originating session id via HERMES_SESSION_ID
with save/restore around the agent loop.
- Tool: kanban_create reads HERMES_SESSION_ID (with explicit override).
- CLI: 'hermes kanban list --session <id>' filter; JSON output exposes
session_id.
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