feat(codex): add Bash PostToolUse graph guidance

[blankanswer]

Summary

• Install a Codex PostToolUse hook for Bash alongside the existing Codex SessionStart / PreToolUse hooks.
• Route Codex PostToolUse Bash events through a small normalization layer when the command is grep-family, then reuse the existing runPostToolUse / postGrep enrichment path.
• Keep Codex soft-only: only emit hookSpecificOutput.additionalContext; no deny, stop, suppress, or input rewrite.
• Preserve user hooks, keep hook installation idempotent, and continue respecting --no-hooks.
• Document the Codex Bash PostToolUse hook.

Design notes

• This intentionally handles only grep-family Bash output.
• Broader Bash output enrichment such as cat / sed / awk / source-read output, apply_patch, and MCP tool names are left for later PRs.
• Codex uses its dedicated dispatcher, so Claude, Gemini, and Antigravity hook behavior remains unchanged.
• The installed hook uses the portable command form: gortex hook --agent=codex --mode=enrich. Unlike the SessionStart text hook, this does not rely on POSIX-only shell output, so this PR does not add a command_windows override.

Out of scope

• apply_patch
• MCP tool-name matching
• cat / sed / awk / general Bash output enrichment
• deny / enforcement behavior
• input rewrite
• output suppression
• Claude / Gemini / Antigravity behavior changes

Tests

• go test ./internal/agents/codex
• go test ./internal/hooks
• go test ./cmd/gortex
• git diff --check

Related to #206