docs: move USAGE.md from docs/ to project root

docs: add comprehensive changelog for v1.3.0 RC1

- Add complete CHANGELOG.md documenting project history from v0.1.0 to v1.3.0-rc1
- Include sections for major features (OAuth refresh, OAuth support, MITM dual-proxy)
- Document new features (statusline integration, enhanced CLI/status output)
- Document bug fixes (health check skip, queue operations)
- Add security notes (.claude/ in .gitignore, debug gate for body logging)
- Add upgrade notes and configuration recommendations
- Propagate debug flag from top-level config to mitm config
- Add debug flag to mitm script initialization
- Add security gate for request body logging (only in debug mode)

v1.2.0

**Keyless API Access**

Multi-provider OAuth support via `oat_sources` configuration. When enabled, the proxy automatically forwards the correct OAuth token based on routing, eliminating the need to manage API keys manually. Includes custom User-Agent support per token source.

**Improved Hooks**

Added hooks that accept parameters. This is compatible with inline hooks and is backwards compatible. See the `ccproxy.yaml` example file for reference.

**LangFuse Integration**

First-class LangFuse observability support. Claude Code sessions are automatically extracted and grouped as LangFuse sessions, with HTTP headers captured as trace metadata (sensitive values redacted). See `.env.example` for setting up your LangFuse credentials.

**CLI Improvements**

The `ccproxy status` command output has been improved to display hooks and model deployments. The handler file (`ccproxy.py`) is now auto-generated on startup, with detection to preserve custom handler files.

- Use bundled litellm from venv instead of PATH (fixes "No module named 'backoff'" error)

- Add installation and troubleshooting guides
- Fix installation commands and prioritize PyPI
- Document auto-generated handler workflow

- Update Opus model references to claude-opus-4-5-20251101
- Update ccproxy.yaml template to current format (oat_sources, capture_headers)
- Comprehensive test coverage improvements (hooks.py 75% → 97%)