## Why
Importing large external-agent session histories currently starts a full
live Codex thread for every imported session. This initializes unrelated
runtime systems and repeats expensive transcript, metadata, hashing, and
ledger work.
On a 50-session, 238 MiB fixture, the existing path took roughly 70
seconds to complete the import and 77 seconds end to end.
## What changed
- Persist imported sessions directly through `ThreadStore` instead of
starting full live threads.
- Process imports through a bounded five-session pipeline.
- Parse, extract, and hash each source file in one pass.
- Move blocking source preparation onto the blocking thread pool.
- Reuse prepared content hashes and update the import ledger once per
batch.
- Avoid metadata readback for newly written rollouts.
- Preserve imported conversation history and visible thread metadata.
- Keep the implementation out of `codex-core` and avoid changes to the
public `ThreadStore` trait.
## Performance
For the same 50-session, 238 MiB fixture:
| Path | Import completion | End to end |
| --- | ---: | ---: |
| Existing import | 69.61s | 76.62s |
| This change | 5.95s | 6.58s |
All 50 sessions imported successfully with no warnings or contention
signals.
## Validation
- `just test -p codex-external-agent-sessions`
- `just test -p codex-app-server external_agent_config_import`
- Verified imports do not initialize unrelated required MCP servers.
- Verified previously imported source versions are skipped and changed
sources can be imported again.
- Verified imported rollouts remain readable through thread listing and
history APIs.
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