Two fixes for issue #133 where setting ANTHROPIC_API_KEY=dummy alongside
CLAUDE_CODE_USE_GEMINI=1 causes "Invalid API key" errors:
1. auth.ts: In the CI branch of getAnthropicApiKeyWithSource(), the
ANTHROPIC_API_KEY value was returned without checking isUsing3PServices().
A dummy key leaked into the Anthropic key resolution pipeline even when
Gemini was the active provider. Now guards with isUsing3PServices().
2. errors.ts: The x-api-key error handler surfaced "Invalid API key" for
any provider. Added getAPIProvider() === 'firstParty' guard so 3P users
see the real underlying error instead of a misleading auth message.
Note: The cli.tsx Gemini validation fix (originally part of this PR) was
independently implemented in PR #121 and is already on main.
Squash the current repository state back into one baseline commit while
preserving the README reframing and repository contents.
Constraint: User explicitly requested a single squashed commit with subject "asdf"
Confidence: high
Scope-risk: broad
Reversibility: clean
Directive: This commit intentionally rewrites published history; coordinate before future force-pushes
Tested: git status clean; local history rewritten to one commit; force-pushed main to origin and instructkr
Not-tested: Fresh clone verification after push
