Move memory prompt injection to app-server extension (#22841)

## Why

Memory prompt injection should be owned by the extension path that
app-server composes at runtime, not by an inlined special case inside
`codex-core`. This keeps `codex-core` focused on session orchestration
while allowing the memories extension to own its app-server prompt
behavior.

## What Changed

- Registers `codex-memories-extension` in the app-server extension
registry.
- Moves the memory developer-instruction injection out of
`core/src/session/mod.rs` and into the memories extension prompt
contributor.
- Adds config-change handling so the extension keeps its per-thread
memory settings in sync after startup.
- Leaves memories read/retrieval tools unregistered for now so this PR
only changes prompt injection.
- Removes the stale `cargo-shear` ignore now that app-server depends on
the extension crate.

## Validation

Not run locally; validation is left to CI.

codex-rs/app-server/src/extensions.rs

@@ -18,6 +18,7 @@ where
 {
     let mut builder = ExtensionRegistryBuilder::<Config>::new();
     codex_guardian::install(&mut builder, guardian_agent_spawner);
+    codex_memories_extension::install(&mut builder);
     Arc::new(builder.build())
 }