feat(ai): add configurable model env vars and ai env flags

[dubscode]

Summary

• add model override env vars for AI provider selection: DUBSTACK_GEMINI_MODEL and DUBSTACK_AI_GATEWAY_MODEL
• extend dub ai env with --gemini-model and --gateway-model, and allow model-only updates
• apply soft validation for model values (non-empty; Gemini model rejects / form)
• use model overrides in both dub ai ask and dub create --ai with existing defaults unchanged
• update README examples and add/expand tests for env config + model resolution paths

Test Plan

• pnpm vitest run src/commands/ai-env.test.ts src/commands/ai.test.ts src/commands/create.test.ts src/lib/history.test.ts
• pnpm test
• pnpm typecheck
• pnpm checks

[Copilot]

Pull request overview

This PR adds configurable AI model environment variables (DUBSTACK_GEMINI_MODEL and DUBSTACK_AI_GATEWAY_MODEL) that allow users to override the model used by both dub ai ask and dub create --ai without changing provider credentials. It also extends dub ai env with --gemini-model and --gateway-model flags for writing model overrides to shell profiles, updates the README with examples, and adds validation (blocking empty or slash-containing Gemini model names via the CLI).

Changes:

• ai-env.ts/ai-env.test.ts: Added model validation functions (normalizeGeminiModel, normalizeGatewayModel) and updated the guard logic to allow model-only updates without keys
• ai.ts/create.ts + their tests: Updated resolveModel in both commands to read DUBSTACK_GEMINI_MODEL/DUBSTACK_AI_GATEWAY_MODEL env vars, falling back to the new defaults (gemini-3-flash-preview and google/gemini-3-flash)
• README.md/index.ts/history.test.ts: Documentation and CLI option registration for the new flags

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/commands/ai-env.ts	Adds model options, validation functions, and updated no-op guard
src/commands/ai-env.test.ts	Tests for model writes, key+model together, empty/slash validation, and updated no-arg error
src/commands/ai.ts	resolveModel now reads DUBSTACK_GEMINI_MODEL/DUBSTACK_AI_GATEWAY_MODEL with fallback defaults
src/commands/create.ts	Same resolveModel update as ai.ts
src/commands/ai.test.ts	Tests for Gemini and Gateway model overrides in askAi; updates default model expectation
src/commands/create.test.ts	Tests for Gemini and Gateway model overrides in create --ai
src/lib/history.test.ts	Confirms --gemini-model / --gateway-model args are not redacted from history
src/index.ts	Registers --gemini-model and --gateway-model options on dub ai env
README.md	Updated provider/key selection docs and added model-override examples

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.