Add Codex CLI agent integration

[jcleira]

Description

Extend Partio's agent detection to recognise OpenAI Codex CLI (codex) as a supported AI agent alongside the existing Claude Code integration.

Partio currently only detects Claude Code via process inspection (internal/agent/claude/). The agent.Detector interface is already designed to be pluggable, so adding Codex CLI follows the same pattern.

What to implement

1. Add a codex package under internal/agent/codex/ with process detection and a Detector implementation
2. Register the detector in the detection chain alongside Claude Code
3. Update partio status to report codex as the agent name when detected
4. Table-driven unit tests for process detection

Why this matters

Codex CLI is a widely used AI coding agent. Users who run Codex to write code and then commit should have those sessions captured by Partio. Without this, Codex-assisted commits are attributed as 100% human work and no session context is preserved.

Source

Inspired by entireio/cli PR#772 and changelog 0.5.2 which added Codex CLI agent integration.

[jcleira]

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[jcleira]

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[jcleira]

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