Add structured debug and warn logging in the post-commit hook to help
users diagnose why commits may not receive Partio-Checkpoint trailers.

- Warn-level logging at each early return explaining why no trailer was
  added (agent inactive, commit already processed, session condensed)
- Debug-level logging of commit file paths and session info for
  diagnosing path mismatch issues (visible with PARTIO_LOG_LEVEL=debug)
- Add git.DiffNameOnly helper to extract changed file paths from commits

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Upgrade early-exit log messages in the post-commit hook from Debug to
Warn level so users running PARTIO_LOG_LEVEL=debug (or default) can
immediately see why no Partio-Checkpoint trailer was added to a commit.

Each exit path now emits a structured "post-commit: no checkpoint created"
warning with the specific reason (no pre-commit state, no active agent,
commit already processed, session already condensed) and relevant context
(commit hash, session ID, state file path).

Also adds a Debug-level log after session lookup showing the resolved
session path, helping diagnose path mismatch issues.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Allow overriding the agent name via PARTIO_AGENT env var, completing
the documented environment variable support in CLAUDE.md.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

## Objective

Detect any running AI agent automatically instead of requiring
PARTIO_AGENT env var to be set to the correct agent name.

## Why

Users running Codex, Claude Code, or any other agent shouldn't
have to configure PARTIO_AGENT. The hooks should detect whichever
agent is active and capture its session.

## How

- Added DetectActive() to agent registry that checks IsRunning()
  on all registered detectors
- Pre-commit hook now auto-detects if no explicit agent is
  configured, or if the configured agent isn't running
- Pre-commit state now includes AgentName so post-commit uses the
  detected agent (not the configured default)
- Post-commit reads agent name from state, falls back to config
- Fixed codex FindSessionDir test to use temp HOME dir

Partio-Checkpoint: 205608af50b1
Partio-Attribution: 100% agent

## Objective

Capture Codex CLI session data in checkpoints and auto-detect
any running agent without requiring PARTIO_AGENT config.

## Why

The Codex integration only detected the process but captured no
session data. The hooks had Claude-specific type assertions that
skipped all other agents. Users shouldn't have to set an env var
to tell Partio which agent they're using.

## How

- Added SessionParser interface to agent package — any detector
  that can provide session data implements FindLatestSession
- Implemented Codex JSONL parser (parse_jsonl.go) that extracts
  session ID, prompt, transcript, token counts from Codex format
- Implemented FindLatestSession for Codex that matches sessions
  to repos by comparing cwd from session_meta
- Replaced all detector.(*claude.Detector) type assertions in
  hooks with detector.(agent.SessionParser) interface checks
- Added DetectActive() to registry that checks all registered
  detectors — hooks auto-detect first, fall back to config
- Pre-commit state now carries AgentName for post-commit
- Default agent config changed from "claude-code" to "" (auto)
- Fixed codex test to use temp HOME dir

Partio-Checkpoint: b34e9e44f11a
Partio-Attribution: 100% agent

## Objective

Stop tracking .partio/sessions/ and .partio/state/ in git.

## Why

These files update on every commit (session tracking, processed
commits list) and create noise in git status.

## How

Added .partio/sessions/ and .partio/state/ to .gitignore and
removed them from tracking.

Partio-Checkpoint: c01093c0919c
Partio-Attribution: 100% agent