fix(ai): bypass v2 SDK session methods for structured output (#110)

[kumar-shivang]

Summary

PR #101 switched structured-output generation in opencode-provider.ts to
client.session.create / client.session.prompt / client.session.delete on
@opencode-ai/sdk v2. On @opencode-ai/sdk@1.14.48 this regresses auto-capture
100% of the time: the only Session class the published bundle exposes at
client.session has shifted across releases (in v1.14.48 it is a thin
experimental class with only list(); the real create/prompt/delete live
on a differently-named class reachable via a different property path), so the
client.session.create(...) call returns undefined and
created?.data?.id is undefined, triggering:

Error: opencode-mem: session.create returned no session id; cannot generate structured output

on every idle event. See #110.

Fix

Stop using the SDK's session methods for these three calls. Hit the
documented server endpoints directly with raw fetch:

action	endpoint
create	POST /session
prompt	POST /session/{id}/message
delete	DELETE /session/{id}

These paths are stable across server versions and match exactly what
tests/opencode-provider.test.ts already mocks, so the existing test suite
is preserved.

The base URL is captured when createV2Client(serverUrl) is called and
cached in a module-level _v2BaseUrl; the client argument on
StructuredOutputOptions is retained for API compatibility but is no
longer used. The setV2Client / getV2Client / createV2Client exports
are preserved so the rest of the codebase (e.g. src/index.ts,
src/services/auto-capture.ts, src/services/user-memory-learning.ts) is
unaffected.

Tests

• bun run typecheck — clean
• bun test — 162 pass / 0 fail (was 154 baseline; +8 new tests)
• bunx prettier --check on changed files — clean

New tests in tests/opencode-provider.test.ts (the
generateStructuredOutput regression tests (issue #110) block) cover what
the previous suite did not:

1. directory is forwarded as a percent-encoded query param on all three
   calls (create, prompt, delete).
2. directory is omitted (no empty ?directory=) when not provided.
3. Base URL with a trailing slash is normalized — no //session double
   slash.
4. createV2Client accepts a URL object (not just a string).
5. Non-2xx response from POST /session surfaces a clear error and skips
   prompt/delete.
6. Non-2xx response from POST /session/{id}/message surfaces the error
   and still runs the best-effort DELETE.
7. Network error (fetch rejection) from POST /session propagates and
   skips prompt/delete.
8. Public API surface (generateStructuredOutput, createV2Client) is
   preserved.

Notes

• The previous implementation happened to pass the existing tests because
  they stub globalThis.fetch, and the v2 SDK's underlying hey-api client
  ultimately calls globalThis.fetch. So the test suite stayed green even
  though the production call path was broken — the fix here is verified
  by reproducing the same globalThis.fetch mock in new tests.
• The issue's suggested fix (option 2: fall back to raw fetch) is what
  this PR implements. Option 1 (wait for the SDK to publish the methods)
  is unnecessary once we talk to the server directly.

Refs

• Closes Auto-capture broken: SDK v1.14.48 has no session.create method #110
• Related: refactor(ai): use opencode v2 SDK session.prompt for structured output #101 (the PR that introduced the regression)