#10346: Add Anthropic to implicit model catalog (Opus 4.6 allowlist)

## Context Users with Anthropic API keys see `/models anthropic` list Opus 4.6, but `/model anthropic/claude-opus-4-6` fails with "model not allowed" unless they manually configure providers. This happens because Anthropic is not included in `resolveImplicitProviders`, so the implicit model catalog does not include Anthropic models by default. ## Changes - Add an implicit Anthropic provider when an Anthropic auth profile or env key exists. - Seed a minimal Anthropic model list (Opus 4.6, Opus 4.5, Sonnet 4.5) so the catalog/allowlist works out of the box. ## Why this fixes the bug The allowlist uses the model catalog. By adding Anthropic to implicit providers, the catalog includes `claude-opus-4-6`, so `/model` selections are allowed without requiring manual provider configuration. ## Notes Costs are set to zero (same pattern as other providers without published pricing). <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> - Adds an implicit Anthropic provider to the runtime model/provider catalog when Anthropic credentials are detected. - Seeds a minimal Anthropic model list (Opus 4.6 / 4.5, Sonnet 4.5) with default context/max tokens and zeroed costs. - Integrates the implicit provider into the same implicit-provider resolution flow used for other providers (e.g., MiniMax/Moonshot). <h3>Confidence Score: 2/5</h3> - This PR is not safe to merge as-is due to a credential wiring bug that will break Anthropic authentication for implicit providers. - The new implicit Anthropic provider populates `apiKey` from the environment *variable name* rather than the secret value, which will lead to runtime auth failures whenever Anthropic is enabled via env vars (a primary intended path for this PR). - src/agents/models-config.providers.ts <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->