agent: Migrate thread storage to SQLite with zstd compression (#31741)

Previously, LMDB was used for storing threads, but it consumed excessive
disk space and was capped at 1GB.

This change migrates thread storage to an SQLite database. Thread JSON
objects are now compressed using zstd.

I considered training a custom zstd dictionary and storing it in a
separate table. However, the additional complexity outweighed the modest
space savings (up to 20%). I ended up using the default dictionary
stored with data.

Threads can be exported relatively easily from outside the application:

```
$ sqlite3 threads.db "SELECT hex(data) FROM threads LIMIT 5;" |
    xxd -r -p |
    zstd -d |
    fx
```

Benchmarks:
- Original heed database: 200MB
- Sqlite uncompressed: 51MB
- sqlite compressed (this PR): 4.0MB
- sqlite compressed with a trained dictionary: 3.8MB


Release Notes:

- Migrated thread storage to SQLite with compression

crates/agent/Cargo.toml

@@ -46,6 +46,7 @@ git.workspace = true
 gpui.workspace = true
 heed.workspace = true
 html_to_markdown.workspace = true
+indoc.workspace = true
 http_client.workspace = true
 indexed_docs.workspace = true
 inventory.workspace = true
@@ -78,6 +79,7 @@ serde_json.workspace = true
 serde_json_lenient.workspace = true
 settings.workspace = true
 smol.workspace = true
+sqlez.workspace = true
 streaming_diff.workspace = true
 telemetry.workspace = true
 telemetry_events.workspace = true
@@ -97,6 +99,7 @@ workspace-hack.workspace = true
 workspace.workspace = true
 zed_actions.workspace = true
 zed_llm_client.workspace = true
+zstd.workspace = true
 
 [dev-dependencies]
 buffer_diff = { workspace = true, features = ["test-support"] }