Add session history listing (`sessions` command)

[roeimichael]

Problem

The status command only shows "active sessions" (events in the last 5 minutes). Once a session ends, there is no CLI command to see it or its stats. Users cannot review what happened in yesterday's sessions — the data is in the database but only accessible via SQLite directly.

What to do

1. Add an API endpoint /api/sessions in routes.py:

   SELECT DISTINCT session_id, MIN(timestamp_ms) as started, 
          MAX(timestamp_ms) as ended, COUNT(*) as event_count 
   FROM events GROUP BY session_id ORDER BY ended DESC LIMIT ?

2. Add a sessions CLI command with optional --days N filter
3. Render a Rich table with: Session ID (truncated), Start Time, End Time, Duration, Event Count, Status (active/ended)

Files to look at

• src/context_analyzer_tool/collector/routes.py — follow pattern of /api/status
• src/context_analyzer_tool/cli.py — follow pattern of anomalies command