Feat/bankr provider (#888)

* feat(provider): add Bankr LLM Gateway support

Add Bankr as an OpenAI-compatible provider preset with dedicated env vars:
- BNKR_API_KEY, BANKR_BASE_URL, BANKR_MODEL
- Uses X-API-Key header instead of Authorization Bearer
- Base URL: https://llm.bankr.bot/v1
- Default model: claude-opus-4.6

Changes:
- Add 'bankr' to VALID_PROVIDERS and provider flag handling
- Add buildBankrProfileEnv() with env key registration
- Add Bankr detection in startup screen and provider discovery
- Map Bankr env vars to OpenAI-compatible vars in shim
- Add Bankr preset to ProviderManager (alphabetical order)
- Update PRESET_ORDER test to include Bankr

Co-Authored-By: OpenClaude <openclaude@gitlawb.com>

* fixup(provider): address Bankr PR review feedback

1. Map BNKR_API_KEY → OPENAI_API_KEY in providerFlag.ts so
   --provider bankr works with BNKR_API_KEY in non-interactive startup.

2. Remove unconditional BANKR_MODEL read from model.ts; it maps to
   OPENAI_MODEL via providerFlag.ts and openaiShim.ts, preventing
   cross-provider leakage.

3. Use X-API-Key for Bankr model discovery in openaiModelDiscovery.ts
   and providerDiscovery.ts, matching chat request auth.

Co-Authored-By: OpenClaude <openclaude@gitlawb.com>

---------

Co-authored-by: OpenClaude <openclaude@gitlawb.com>

src/utils/providerSecrets.ts

@@ -4,6 +4,7 @@ const SECRET_ENV_KEYS = [
   'GEMINI_API_KEY',
   'GOOGLE_API_KEY',
   'MISTRAL_API_KEY',
+  'BNKR_API_KEY',
 ] as const
 
 export type SecretValueSource = Partial<