feat(timeline): structured session timeline view (#81)

[Siddhant-K-code]

What

Adds agent-strace timeline — a structured, phase-by-phase view of a session showing tool calls, file operations, LLM requests, errors, retries, and a wasted-spend callout for failed phases.

Closes #81

Example

Session: abc123def456 | 2026-05-19 14:32 | 4m 12s | $0.0043 | 3 errors

Phase 1: Setup (0:00–0:45)  $0.0008
  ✓ Read src/auth/handler.go
  ✓ Read src/auth/middleware.go

Phase 2: Implementation (0:45–2:10)  $0.0031
  · Run Bash (1.2s)
      pytest tests/test_auth.py
  ✗ Error: Bash
      FAILED tests/test_auth.py::TestAuthHandler
  · Run Bash (attempt 2) (1.1s)
  ✓ Write src/auth/handler.go  +3 lines

  ⚠ 2 retries in this phase

⚠ Wasted spend: 2 retries on failed phases = ~$0.0008 (19% of session cost)

Changes

• src/agent_trace/timeline.py — new module (build_timeline, format_timeline, format_timeline_json, cmd_timeline)
• src/agent_trace/cli.py — registers timeline subparser
• tests/test_timeline.py — 66 tests
• README.md — new section + command reference entries
• src/agent_trace/__init__.py — version 0.50.0 → 0.51.0

Design notes

• Reuses explain.py for phase detection and cost.py for token/cost estimation — no new dependencies
• Retry detection: same tool+args key seen more than once in a phase
• --format json emits the full phase/entry tree for downstream tooling
• --model selects pricing table (sonnet/opus/haiku/gpt4/gpt4o)