feat: Multi-agent support with adapter pattern + Claude Code integration

[mazzucci]

Summary

Refactors the monolithic flush.py into a core engine + adapter pattern to support multiple coding agents with consistent trace output in Phoenix.

What changed

File	Change
hooks/core.py	New agent-agnostic engine: NormalizedEvent dataclass, turn assignment, span building, Phoenix posting
hooks/adapters/cursor.py	Cursor adapter: buffer I/O, atomic drain, hook event normalisation
hooks/adapters/claude_code.py	Claude Code adapter: JSONL transcript parser, tool_use/result pairing, thinking blocks
hooks/flush.py	New entrypoint with --agent flag dispatching to adapters
hooks/claude-code-trace-hook.sh	Claude Code Stop hook script
install.sh	Auto-detects Cursor + Claude Code, installs hooks for each
docker-compose.yml	Exposes gRPC port 4317 for OTel collector
.github/workflows/ci.yml	CI: Python 3.10-3.13 tests, lint, shellcheck
tests/test_flush.py	35 tests (was 21): core engine, both adapters, cross-adapter parity
README.md	Multi-agent docs, architecture diagram, badges

Architecture

hooks/
├── flush.py                       # Entrypoint: --agent cursor|claude_code
├── core.py                        # NormalizedEvent → turns → spans → Phoenix
├── trace-hook.sh                  # Cursor hook (unchanged)
├── claude-code-trace-hook.sh      # Claude Code Stop hook
└── adapters/
    ├── __init__.py                # Adapter registry
    ├── cursor.py                  # Cursor: buffer I/O + normalisation
    └── claude_code.py             # Claude Code: transcript parser

Key design decisions

• Both adapters produce identical span structures via NormalizedEvent → consistent Phoenix traces regardless of agent
• Claude Code uses Stop hook + transcript parsing (not OTel native) for trace-level parity with Cursor
• Backward compatible: CURSOR_TRACES_* env vars still work
• Extensible: adding a new agent = one adapter file + registry entry

Tests

All 35 tests pass:

• 20 core engine tests
• 6 Cursor adapter tests
• 8 Claude Code adapter tests
• 1 cross-adapter parity test