docs: document Claude Code opt-out

[Indosaram]

Summary

• Document the existing disabledBuiltinExtensions opt-out for the claude-sdk-oauth builtin.
• Explain that disabling claude-sdk-oauth prevents the Claude Code provider extension from loading while preserving other builtins.
• Record the configuration-only change in the extension changelog.

Verification

• npm exec vitest -- --run test/claude-sdk-oauth-options.test.ts test/suite/claude-sdk-oauth-extension.test.ts ✅ (44 tests)
• node .agents/skills/senpi-qa/scripts/cli-smoke.mjs --self-test ✅ (8/8)
• git diff --check ✅
• npm run check ⚠️ reaches repository checks, but the existing packages/web-ui check fails because lit is unavailable in the workspace environment; no changed-file errors were reported.

Closes no issue.

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Summary by cubic

Document how to opt out of the Claude Code provider by disabling the claude-sdk-oauth builtin via disabledBuiltinExtensions. Adds a JSON example, restart and precedence notes (project overrides global), updates the extension changelog, and includes a regression test confirming the provider is skipped while other built-ins remain loaded.

^{Written for commit 379f0e6. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}