feat(codex): add Bash PreToolUse graph guidance

[blankanswer]

Summary

Adds a Codex-specific Bash PreToolUse hook that nudges shell search/read commands toward Gortex graph tools in soft enrich mode only.

Changes

• Install a Codex PreToolUse hook for ^Bash$ alongside the existing SessionStart hook.
• Route gortex hook --agent=codex through a Codex-specific dispatcher.
• Force Codex PreToolUse handling through ModeEnrich, so it emits hookSpecificOutput.additionalContext without denying tool calls.
• Preserve existing user Codex hooks and keep repeated installs idempotent.
• Replace only Gortex-owned Codex hook entries under --force.
• Document Codex SessionStart plus Bash-only soft PreToolUse behavior.

Design notes

From the Codex hook behavior spike:

• Codex can consume hookSpecificOutput.additionalContext, so this uses the existing HookOutput JSON shape instead of plain stdout guidance.
• The installed hook command includes --mode=enrich for readability, but Codex handling is clamped in code: RunCodex always calls runPreToolUse(..., ModeEnrich) and does not read the CLI --mode flag. This keeps Codex soft-only even if the hook command is hand-edited.
• This PR adds only the portable command form for the Codex PreToolUse hook. Unlike SessionStart, this command just invokes gortex hook; it does not rely on POSIX-only shell text like printf. A Windows-specific command_windows override can be added later if native Windows hook behavior needs it.
• Codex uses a dedicated dispatcher rather than extending RunExternalAgent, keeping Gemini / Antigravity lifecycle hook behavior unchanged.

Testing

• New tests added for new functionality
• Benchmarks run if performance-relevant (not performance-relevant)

Ran:

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

Checklist

• Code follows existing patterns in the codebase
• No unnecessary abstractions added
• Language extractor includes Meta["methods"] for interfaces (if applicable)
• Methods have EdgeMemberOf edges to their containing type (if applicable)

Related to #206