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/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,