[codex] Avoid duplicate hooks.json discovery with profiles

[abhinav-oai]

Summary

V2 profiles add both config.toml and <profile>.config.toml to the config stack. Because both user layers resolve hook discovery to the same Codex home, Codex loaded the same hooks.json twice. This duplicated hook rows and caused each matching command to run twice.

Deduplicate JSON hook discovery by absolute config folder within each effective config stack. TOML hooks remain layer-specific, and multi-cwd hooks/list results remain independently resolved per cwd.

Reproduction

1. Add config.toml and work.config.toml under $CODEX_HOME.
2. Add one command hook to $CODEX_HOME/hooks.json.
3. Run Codex with --profile work.
4. Trigger the hook.

Before this change, one declaration creates two handlers. Afterward, it creates one.

Fixes #25645 and addresses the single-cwd duplication in #25437.

Validation

• cargo nextest run -p codex-hooks
• just fix -p codex-hooks
• just fmt
• just argument-comment-lint -p codex-hooks