* refactor: replace persistent subagents with ephemeral spawn tool (#subagent)
- Drop subagents table, remove all persistent subagent infrastructure
- Add 'subagent' session type with parent_session_id on bot_sessions
- Rewrite subagent tool as single 'spawn' tool with parallel execution
- Create system_subagent.md prompt, add _subagent.md include for chat
- Limit subagent tools to file, exec, web_search, web_fetch only
- Merge subagent token usage into parent chat session in reporting
- Remove frontend subagent management page, update chat UI for spawn
- Fix UTF-8 truncation in session title, fix query not passed to agent
* refactor: remove history message page
- Rewrite SQL queries to join bot_history_messages with bot_sessions,
supporting chat/heartbeat/schedule usage from a single source
- Update Go handler and CLI command to use unified queries
- Fix daily chart stacking: each session type gets its own bar group
- Add total input/output trend lines to the daily token chart
- Fix summary cards reactivity by restricting aggregation to allDays range
- Fix cache chart reactive dependency tracking by inlining data access
- Add i18n keys for schedule, totalInput, totalOutput
- Default time range changed to 7 days
- Regenerate sqlc, swagger, and SDK
Acbox Liu