External agent session support (#19895)

## Summary

This extends external agent detection/import beyond config artifacts so
Codex can detect recent sessions files from the external agent home and
import them into Codex rollout history.

## What changed

- Added a focused `external_agent_sessions` module for:
  - session discovery
  - source-record parsing
  - rollout construction
  - import ledger tracking
- Wired session detection/import into the app-server external agent
config API.
- Added compaction handling so large imported sessions can be resumed
safely before the first follow-up turn.

## Testing

Added coverage for:
- recent-session detection
- custom-title handling
- recency filtering
- dedupe and re-detect-after-source-change behavior
- visible imported turn construction
- backward-compatible import payload deserialization
- end-to-end RPC import flow
- rejection of undetected session paths
- repeat-import behavior
- large-session compaction before first follow-up

Ran:
- `cargo test -p codex-app-server external_agent_config_import_ --test
all`

codex-rs/Cargo.lock

@@ -1853,6 +1853,7 @@ dependencies = [
  "codex-core-plugins",
  "codex-device-key",
  "codex-exec-server",
+ "codex-external-agent-sessions",
  "codex-features",
  "codex-feedback",
  "codex-file-search",
@@ -2682,6 +2683,20 @@ dependencies = [
  "syn 2.0.114",
 ]
 
+[[package]]
+name = "codex-external-agent-sessions"
+version = "0.0.0"
+dependencies = [
+ "chrono",
+ "codex-app-server-protocol",
+ "codex-protocol",
+ "codex-utils-output-truncation",
+ "serde",
+ "serde_json",
+ "sha2",
+ "tempfile",
+]
+
 [[package]]
 name = "codex-features"
 version = "0.0.0"