fix: preconfigured-client example and singleton client manager dispose recovery

[ben-vargas]

Summary

Fixes the client-options.ts example, which claimed to demonstrate the preconfigured-client pattern but did not, and fixes the underlying client-manager lifecycle issue uncovered while verifying it.

Example: step 2 never used the preconfigured client

createOpencode({ client }) hands the client to the process-wide singleton OpencodeClientManager, which step 1 had already initialized. The singleton warned and kept using step 1's client, so step 2's requests carried step 1's x-demo-source: client-options-example header while stdout misleadingly printed a success response.

• Step 2 now passes an isolated manager: createOpencode({ clientManager: OpencodeClientManager.createInstance({ client }) }), with a comment explaining why.
• Both clients wrap fetch to echo each outgoing request's x-demo-source header, so the demo output itself proves which client served each request.
• Dropped the spurious await on createOpencodeClient() (synchronous in @opencode-ai/sdk/v2).
• The example model is overridable via OPENCODE_MODEL (default unchanged, matching the other examples).

Client manager: dispose() bricked future providers

dispose() set isDisposed but left the static singleton pointing at the dead instance, so any later createOpencode() without an explicit clientManager received the disposed manager and threw Client manager has been disposed on first use. This also made the existing warnings' advice ("call dispose() first") incorrect.

• dispose() now releases the singleton slot when the disposed manager is the singleton, so the next getInstance() builds a fresh manager. Non-singleton managers from createInstance() are unaffected.
• The "already initialized" warnings now recommend the escape hatches that actually work: createInstance() with the clientManager setting, or resetInstance().
• Added a regression test covering getInstance → getClient → dispose → getInstance with new options → getClient succeeds.

Also bumps to 3.0.5 with CHANGELOG entries, following the per-fix release convention.

Testing

• npm run ci passes (typecheck, lint, 359 tests including the new regression test).
• Live against opencode serve --hostname 127.0.0.1 --port 4096:
  • npx tsx examples/client-options.ts — header echo confirms step 1's requests carry x-demo-source: client-options-example and step 2's carry x-demo-source: preconfigured-client-example.
  • Dispose-recovery scenario: provider → generateText → dispose → new createOpencode({ baseUrl }) → generateText succeeds (previously threw Client manager has been disposed).
  • Old broken pattern (createOpencode({ client }) after singleton init) emits the updated warning with the corrected guidance.