feat: add OpenAI Codex CLI agent support

[rohithreddy1095]

Summary

• Adds a new agent plugin for OpenAI Codex CLI, enabling Entire to track Codex sessions alongside Claude Code and Gemini CLI
• Implements the full Agent interface (13 methods) plus optional HookSupport, HookHandler, TranscriptAnalyzer, and TranscriptChunker interfaces
• Follows the existing plugin architecture pattern established by claudecode and geminicli packages
• Includes 46 unit tests across 3 test files

Motivation

Developers increasingly use multiple AI coding agents (Claude Code, Gemini CLI, Codex CLI) in the same repository. Currently Entire only tracks Claude Code and Gemini CLI sessions. This PR adds Codex CLI support so that all three agents' sessions are tracked with unified checkpoint creation.

Implementation Details

New Files

File	Purpose
codexcli/types.go	Codex-specific types: JSONL event types, notify payload struct, transcript line types, file modification heuristics
codexcli/codex.go	Full Agent interface with init() self-registration, session resolution via date-based directory tree, JSONL transcript I/O
codexcli/hooks.go	Hook install/uninstall — manages notify array in ~/.codex/config.toml (TOML string manipulation)
codexcli/transcript.go	JSONL parsing with 10MB scanner buffer, user/assistant message extraction, heuristic file modification detection from exec_command calls, token usage calculation
codexcli/codex_test.go	Unit tests for Agent interface methods and hook input parsing (12 tests)
codexcli/hooks_test.go	Unit tests for hook install/uninstall and TOML config manipulation (12 tests)
codexcli/transcript_test.go	Unit tests for transcript parsing, file extraction, token usage, and command detection (22 tests)
hooks_codexcli_handlers.go	handleCodexTurnComplete() — session dir setup, metadata extraction, strategy-based commit

Modified Files

File	Change
registry.go	Added AgentNameCodex and AgentTypeCodex constants
hook_registry.go	Registered turn-complete handler + codexcli import
hooks_cmd.go	Added blank import for agent registration
summarize/summarize.go	Added AgentTypeCodex to exhaustive switch
setup.go	Added Codex hook removal in removeAgentHooks()

Key Design Decisions

• Single hook: Codex only supports notify (agent-turn-complete), so the handler combines state capture + commit in one pass (unlike Claude's 7 hooks or Gemini's 10 hooks)
• TOML config: Codex uses ~/.codex/config.toml with a notify array, handled via string manipulation (no new TOML dependency)
• File modification detection: Codex uses exec_command with shell commands instead of structured tools. Detection is heuristic-based (handles apply_patch diffs, cat >, sed -i, etc.)
• Session format: JSONL at ~/.codex/sessions/<year>/<month>/<day>/rollout-<date>-<uuid>.jsonl

Test plan

• go build ./... passes
• go vet ./... passes
• golangci-lint passes (0 issues across entire project)
• 46 unit tests pass across codex_test.go, hooks_test.go, transcript_test.go
• gofmt clean
• entire enable --agent codex installs notify hook in ~/.codex/config.toml
• entire enable --agent codex is idempotent (detects already installed)
• entire disable --uninstall removes Codex notify hook and preserves other config
• Codex turn-complete fires and creates checkpoint with metadata
• Existing Claude Code and Gemini CLI hooks continue working (no regressions)