feat(adapter): import Zed Agent threads

[Airine]

Upstream baseline

• Repository: https://github.com/zed-industries/zed
• Version/commit: v1.17.2 / 770aac345c76d9fcf82c01ba0dea08e5739b27b1
• Native storage: Zed's agent threads.db SQLite database in the platform application-support data directory
• Format entry point: crates/agent/src/db.rs

The fixture and implementation must remain pinned to this baseline. If upstream has changed, update the pin and document the schema delta before merging.

v1 import scope

Discover and import compressed native thread data, thread metadata, messages/content, tool operations, usage/context, and unknown data from a WAL-safe snapshot.

This is import-only. Do not add an exporter until upstream exposes a stable, public restore/resume contract and it can be proven with native round-trip and real resume tests.

Do not re-import external ACP-backed conversations already captured through the ACP tap; define and test the native/external identity boundary.

Dependency

• Adapter registry and conformance kit: refactor(usl-convert): add a harness adapter registry and conformance kit #1

Fixture and claiming process

1. Comment with the exact upstream version/commit, fixture provenance, and intended scope.
2. A maintainer confirms scope and assigns the issue before implementation starts.
3. Fixtures must be synthetic or irreversibly sanitized: remove credentials/tokens, usernames, absolute home paths, repository remotes, proprietary prompts/code, model ids when sensitive, and tool output secrets.
4. Preserve native structure, ordering, ids/relationships, encodings, and unknown record shapes; replace sensitive values deterministically and include a provenance README.

Acceptance criteria

• Adapter descriptor, discovery/import registration, and native format metadata are added.
• Import is deterministic and produces a valid ASP bundle.
• Original source files remain byte-identical; the adapter never writes into native storage.
• Malformed records fail or degrade according to the conformance contract.
• Unknown records/fields and opaque payloads survive as evidence rather than being discarded.
• Content, tools, thinking, usage, identity, ordering, and attachments each declare fidelity and explicit loss.
• A sanitized fixture representing the real native structure covers the supported v1 surface.
• SQLite fixtures include database + WAL/SHM behavior; tests copy them safely and prove the live database is unchanged.
• CLI discovery/import, list-formats, support matrix, tests, and architecture docs are updated.
• No exporter is advertised or implemented in v1.
• npm run check and npm run lint:architecture pass; relevant workspace tests remain green.