extension: move git attribution into an extension (#21738)

## Why

Git commit attribution is prompt policy, not session orchestration.
After #21737 adds the extension-registry seam, this moves that
prompt-only behavior out of `codex-core` so `Session` can consume
extension-contributed prompt fragments instead of owning a one-off
policy path itself.

Before this PR, `Session` injected the trailer instruction directly from
`codex-core` ([session
assembly](https://github.com/openai/codex/blob/a57a747eb667753118217b8bb47dfd1fff88cbde/codex-rs/core/src/session/mod.rs#L2733-L2739),
[helper
module](https://github.com/openai/codex/blob/a57a747eb667753118217b8bb47dfd1fff88cbde/codex-rs/core/src/commit_attribution.rs#L1-L33)).
This branch moves that same responsibility into
[`codex-git-attribution`](https://github.com/openai/codex/blob/b5029a67360fe5c948aa849d4cf65fd2597ebaae/codex-rs/ext/git-attribution/src/lib.rs#L14-L100).

## What changed

- Added the `codex-git-attribution` extension crate.
- Snapshot `CodexGitCommit` plus `commit_attribution` at thread start,
then contribute the developer-policy fragment through the extension
registry.
- Register the extension in app-server thread extensions.
- Remove the old `codex-core` helper module and direct `Session`
injection path.

This keeps the existing behavior intact: the prompt is only contributed
when `CodexGitCommit` is enabled, blank attribution still disables the
trailer, and the default remains `Codex <noreply@openai.com>`.

## Stack

- Stacked on #21737.

codex-rs/core/src/commit_attribution.rs

@@ -1,33 +0,0 @@
-const DEFAULT_ATTRIBUTION_VALUE: &str = "Codex <noreply@openai.com>";
-
-fn build_commit_message_trailer(config_attribution: Option<&str>) -> Option<String> {
-    let value = resolve_attribution_value(config_attribution)?;
-    Some(format!("Co-authored-by: {value}"))
-}
-
-pub(crate) fn commit_message_trailer_instruction(
-    config_attribution: Option<&str>,
-) -> Option<String> {
-    let trailer = build_commit_message_trailer(config_attribution)?;
-    Some(format!(
-        "When you write or edit a git commit message, ensure the message ends with this trailer exactly once:\n{trailer}\n\nRules:\n- Keep existing trailers and append this trailer at the end if missing.\n- Do not duplicate this trailer if it already exists.\n- Keep one blank line between the commit body and trailer block."
-    ))
-}
-
-fn resolve_attribution_value(config_attribution: Option<&str>) -> Option<String> {
-    match config_attribution {
-        Some(value) => {
-            let trimmed = value.trim();
-            if trimmed.is_empty() {
-                None
-            } else {
-                Some(trimmed.to_string())
-            }
-        }
-        None => Some(DEFAULT_ATTRIBUTION_VALUE.to_string()),
-    }
-}
-
-#[cfg(test)]
-#[path = "commit_attribution_tests.rs"]
-mod tests;

codex-rs/core/src/commit_attribution_tests.rs

@@ -1,43 +0,0 @@
-use super::build_commit_message_trailer;
-use super::commit_message_trailer_instruction;
-use super::resolve_attribution_value;
-
-#[test]
-fn blank_attribution_disables_trailer_prompt() {
-    assert_eq!(build_commit_message_trailer(Some("")), None);
-    assert_eq!(commit_message_trailer_instruction(Some("   ")), None);
-}
-
-#[test]
-fn default_attribution_uses_codex_trailer() {
-    assert_eq!(
-        build_commit_message_trailer(/*config_attribution*/ None).as_deref(),
-        Some("Co-authored-by: Codex <noreply@openai.com>")
-    );
-}
-
-#[test]
-fn resolve_value_handles_default_custom_and_blank() {
-    assert_eq!(
-        resolve_attribution_value(/*config_attribution*/ None),
-        Some("Codex <noreply@openai.com>".to_string())
-    );
-    assert_eq!(
-        resolve_attribution_value(Some("MyAgent <me@example.com>")),
-        Some("MyAgent <me@example.com>".to_string())
-    );
-    assert_eq!(
-        resolve_attribution_value(Some("MyAgent")),
-        Some("MyAgent".to_string())
-    );
-    assert_eq!(resolve_attribution_value(Some("   ")), None);
-}
-
-#[test]
-fn instruction_mentions_trailer_and_omits_generated_with() {
-    let instruction = commit_message_trailer_instruction(Some("AgentX <agent@example.com>"))
-        .expect("instruction expected");
-    assert!(instruction.contains("Co-authored-by: AgentX <agent@example.com>"));
-    assert!(instruction.contains("exactly once"));
-    assert!(!instruction.contains("Generated-with"));
-}

codex-rs/core/src/lib.rs

@@ -27,7 +27,6 @@ mod agent;
 mod attestation;
 mod codex_delegate;
 mod command_canonicalization;
-mod commit_attribution;
 pub mod config;
 pub mod connectors;
 pub mod context;

codex-rs/core/src/session/mod.rs

@@ -16,7 +16,6 @@ use crate::agent::agent_status_from_event;
 use crate::agent::status::is_final;
 use crate::attestation::AttestationProvider;
 use crate::build_available_skills;
-use crate::commit_attribution::commit_message_trailer_instruction;
 use crate::compact;
 use crate::config::ManagedFeatures;
 use crate::config::resolve_tool_suggest_config_from_layer_stack;
@@ -2712,13 +2711,6 @@ impl Session {
         {
             developer_sections.push(plugin_instructions.render());
         }
-        if turn_context.features.enabled(Feature::CodexGitCommit)
-            && let Some(commit_message_instruction) = commit_message_trailer_instruction(
-                turn_context.config.commit_attribution.as_deref(),
-            )
-        {
-            developer_sections.push(commit_message_instruction);
-        }
         for contributor in self.services.extensions.context_contributors() {
             for fragment in contributor.contribute(
                 &self.services.session_extension_data,