agent: Use inventory for AgentPreview (#30740)

This PR updates the `AgentPreview` to use `inventory` instead of
`linkme`.

Release Notes:

- N/A

crates/agent/Cargo.toml

@@ -47,12 +47,12 @@ heed.workspace = true
 html_to_markdown.workspace = true
 http_client.workspace = true
 indexed_docs.workspace = true
+inventory.workspace = true
 itertools.workspace = true
 jsonschema.workspace = true
 language.workspace = true
 language_model.workspace = true
 language_model_selector.workspace = true
-linkme.workspace = true
 log.workspace = true
 lsp.workspace = true
 markdown.workspace = true