ax: implement monitor command and conversations API

[anj-s]

Summary

• AX Monitor CLI Command: Added the monitor command to the ax CLI, registered in cmd/ax/main.go.
• Backend API: Implemented a local HTTP API endpoint /api/conversations that queries the SQLite database to fetch and summarize AX conversation histories.
• SQLite Parsing Fixes: Handled sqlite timestamps by scanning them as strings and parsing them manually in Go to avoid scan errors when using SQLite aggregations.
• Unit Tests: Added cmd/ax/monitor_test.go to verify correctness of conversation fetching logic.

Type of Change

• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Refactor
• Documentation
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Related Issues

Part 4 of the Observability & Monitoring epic. Stacked on top of PR #70.

Test Plan

• Automated Tests: Unit tests pass:

  go test -v ./cmd/ax

• Manual Verification: Verified that command compilation works both with and without the harness build tag:

  go build -o bin/ax ./cmd/ax
  go build -tags harness -o bin/ax_harness ./cmd/ax

Notes for Reviewer

This is the fourth of 5 planned modular PRs to roll out comprehensive AX observability.
It exposes a backend JSON API for conversation lists.

[rakyll]

See my comment at #75. We shouldn't build this in AX.

[rakyll]

We chatted offline that we need an intermediate solution for introspection when developing.

Let's call this "dashboard", otherwise LGTM.