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/core/src/session/mod.rs

@@ -2690,14 +2690,6 @@ impl Session {
         {
             developer_sections.push(developer_instructions.to_string());
         }
-        // Add developer instructions for memories.
-        if turn_context.features.enabled(Feature::MemoryTool)
-            && turn_context.config.memories.use_memories
-            && let Some(memory_prompt) =
-                build_memory_tool_developer_instructions(&turn_context.config.codex_home).await
-        {
-            developer_sections.push(memory_prompt);
-        }
         // Add developer instructions from collaboration_mode if they exist and are non-empty
         if turn_context.config.include_collaboration_mode_instructions
             && let Some(collab_instructions) =
@@ -3438,8 +3430,6 @@ pub(crate) fn emit_subagent_session_started(
     });
 }
 
-use codex_memories_read::build_memory_tool_developer_instructions;
-
 /// Builds the hook engine for one config snapshot, including any enabled plugin hooks.
 async fn build_hooks_for_config(
     config: &Config,