Feature: Advanced Query Explorer for Events, Agents, and Sessions

## Problem
Power users need more than fixed filters; they need expressive querying across session/agent/event dimensions to answer custom questions quickly.

## Proposal
Add an **Advanced Query Explorer** with a safe query DSL and visual results.

## Scope
- Query builder UI (fields, operators, time ranges, group by)
- Saved queries with tags and sharing
- Result views: table, time-series, aggregation cards
- Export query results (CSV/JSON)

## Query Examples
- Events where `tool_name = Bash` and status became `error`
- Sessions with > N subagents and duration > X minutes
- Agents with repeated reconnect patterns

## Technical Notes
- Implement validated DSL -> SQL translation with strict allowlist
- Add query execution limits/timeouts to protect server
- Index recommendations surfaced from slow query telemetry

## Acceptance Criteria
- [ ] Users can build and run custom queries without SQL access
- [ ] Query validation prevents unsafe operations
- [ ] Saved queries persist and can be rerun/editable
- [ ] Results support sorting, pagination, and export
- [ ] Slow query warnings include optimization hints