[FEATURE]: v2 API gaps blocking third-party clients

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Describe the enhancement you want to request

I maintain a third-party Rust TUI client for opencode and have been porting it to the v2 server. Five capabilities that v1 exposed have no v2 equivalent, and each blocks a client feature rather than being cosmetic. Checked against the published v2 API docs (https://opencode.ai/v2/docs/api) and packages/protocol/openapi.json at a5d6969f.

1. Symbol search. GET /api/fs/find accepts type of file or directory only. v1 had /find/symbol, which backed symbol lookup in the file picker.

2. Per-session diff. Only GET /api/vcs/diff exists and it is workspace scoped (mode is required, there is no session parameter). v1 had /session/{id}/diff, which backed a per-session changed-files view.

3. Provider connection state. GET /api/provider returns id, name, package, settings, integrationID, disabled. A client cannot tell which providers are actually connected, so it cannot show connected state or gate the model picker on it.

4. Model capability flags. GET /api/model capabilities carry tools, input, output only. v1 also exposed reasoning and temperature, which drive badges and filtering in a model picker.

5. Protocol version. server.connected carries an untyped data field and /experimental/capabilities is gone, so a client cannot detect the server protocol version at connect time. That forces a hard cutover instead of supporting v1 and v2 behind detection.

Happy to split these into separate issues if that is easier to track, and happy to be told any of them are intentional removals.

Environment: opencode2 0.0.0-next-17002 on Linux, talking to the HTTP API directly rather than using the bundled TUI.