[codex] migrate Codex preset to hooks and shared bash recovery

[svarlamov]

Summary

• migrate the Codex preset from legacy notify wiring to Codex hooks (PreToolUse, PostToolUse, Stop)
• reuse the shared bash snapshot architecture for inflight agent recovery instead of a Codex-only sidecar path
• keep Codex-specific behavior only where runtime gaps still require it: snapshot-only PreToolUse for Bash and Stop fallback for apply_patch/file-edit turns

Why

Codex now supports bash pre/post hooks, but it still does not emit hooks for file-edit tool calls. The old notify-based path needed to be removed, while commit-time attribution still needed to recover the owning Codex session when git commit happens inside a live bash invocation.

This change moves that recovery into the shared bash snapshot flow so other agents can use the same mechanism, and leaves Codex-specific logic only for the current runtime gaps.

User impact

• Codex installs hooks instead of git-ai notify
• bash-originated Codex edits and commits recover proper AI attribution from active session context
• pure Codex file-edit turns still get unscoped AI attribution through Stop until Codex exposes file-edit hooks
• the shared bash prehook path is simpler and reused across Claude, Gemini, Continue CLI, Amp, OpenCode, Droid, and Firebender

Validation

• cargo fmt -- --check
• git diff --check
• cargo test pre_commit_checkpoint_context_uses_inflight_bash_agent_context -- --nocapture
• cargo test mdm::agents::codex::tests -- --nocapture
• cargo test --package git-ai --test integration codex -- --nocapture
• cargo test --package git-ai --test integration bash_tool_conformance -- --nocapture
• cargo test --package git-ai --test integration test_amp_preset_pretooluse_returns_human_checkpoint -- --nocapture
• cargo test --package git-ai --test integration test_opencode_preset_pretooluse_returns_human_checkpoint -- --nocapture
• cargo test --package git-ai --test integration test_droid_preset_pretooluse_returns_human_checkpoint -- --nocapture
• cargo test --package git-ai --test integration test_firebender_pre_tool_use_maps_to_human_checkpoint -- --nocapture
• cargo test --package git-ai --test integration test_continue_cli_preset_human_checkpoint -- --nocapture
• cargo test --package git-ai --test integration test_gemini_preset_human_checkpoint -- --nocapture
• cargo test --package git-ai --test integration test_claude_preset_pretooluse_checkpoint -- --nocapture
• cargo test --package git-ai --test integration test_gemini_preset_bash_tool_aftertool_detects_changes -- --nocapture

Stack

Base branch: johnw/bash-support
Intended follow-up into: #798

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