Fix Codex PostToolUse hooks that emit empty stdout for pathless edits

[JordanCoin]

Problem

Codex invokes the registered PostToolUse hook after apply_patch, Edit, and Write. Codemap currently returns success with empty stdout when the hook payload has no recognized file_path (notably pathless apply_patch payloads). Codex requires a valid JSON hook response and reports hook returned invalid post-tool-use JSON output.

Reproduction

From the repository root:

printf "%s\n" "{\"tool_name\":\"apply_patch\",\"tool_input\":{},\"tool_response\":{}}" | codemap hook post-edit --agent=codex --integration=codex-setup

Expected current observation: exit 0 with empty stdout.

TDD acceptance criteria

• Add a failing regression test for a Codex PostToolUse invocation whose payload yields no file paths.
• Codex-mode post-edit always emits exactly one valid JSON hook envelope on stdout, including a safe no-op result.
• A file-path payload still records provenance and returns its existing contextual output inside the JSON envelope.
• Hook stdout contains no non-JSON prefixes, ANSI output, or extra JSON records.
• Preserve Claude-mode hook behavior.
• Focused hook tests and go test ./... pass.

Likely scope

cmd/hooks.go and adjacent hook tests only. Do not edit user-wide ~/.codex/hooks.json as a workaround.