fix(state): degrade list_sessions_rich on stale read-only profile DBs

[JuizSpeaking]

Summary

The desktop sidebar (GET /api/profiles/sessions/sidebar) aggregates every profile's state.db read-only, and read-only opens deliberately skip schema reconciliation. A dormant profile's DB can therefore be older than the running binary, missing columns the list queries select unconditionally (last_activity_at, last_read_at, pinned, …). The sidebar's per-profile try/except then swallows the resulting no such column error and silently drops the whole profile from the session list — which reads as "all sessions are gone except the active profile's."

Fix

list_sessions_rich now probes the live sessions columns once per SessionDB instance and degrades instead of raising on stale-schema read-only opens:

• Activity expression (_sql_session_last_active / _by_id): falls back to MAX(messages.timestamp) + started_at when last_activity_at is absent. The UNION ALL keeps a NULL arm so the MAX subquery stays well-formed.
• Compact projection (_compact_session_cols_live): intersected with the live columns, so compact_rows=True never selects a column the DB doesn't have.
• Pinned back-fill: skipped when the pinned column is absent.
• Compression-tip batch fetch (_get_session_rich_rows_batch): follows the same guard since it backs list_sessions_rich's projection.

Writable opens are unaffected: they still reconcile the schema at startup, so the probe always sees the full column set there.

Tests

New tests/test_hermes_state_readonly_stale_schema.py:

• compact_rows listing against a genuinely stale read-only DB returns rows (degraded last_active) instead of raising
• fresh-schema DBs still satisfy the declared projection contract
• _compact_session_cols_live intersection behavior

Focused suite: 216 passed (state, readonly-preflight, WAL-fallback + new file).

Repro

db = SessionDB(db_path=stale_db, read_only=True)
db.list_sessions_rich(compact_rows=True, order_by_last_active=True, include_pinned=True)
# before: OperationalError: no such column: last_activity_at
# after:  rows with last_active derived from messages/started_at

[jquesnelle]

Appreciate the contribution! Closed by #80797