feat(providers): formalize model discovery modes

[Lxr-max]

Summary

Provider model discovery still mostly assumed a listing endpoint, even though real providers split into GET /models, a static catalog, manual IDs, inference-only gateways, and imports with a single known default. This adds a formal modelDiscoveryMode and drives listing, Settings pickers, import defaults, connection tests, and diagnostics from it so “no /models” is no longer a uniform hard failure.

Fixes #210

Mode enum: models | static-hint | manual | infer-only

Mode	models:v1:list-for-provider	Settings picker	Connection test
models	GET /models	remote select	probe /models; 404 + working inference stays degraded
static-hint	local modelsHint + defaultModel	local select	skip /models (ChatGPT Codex OAuth still uses its existing token check)
manual	[defaultModel] locally	text input	skip /models, probe inference → compatible
infer-only	same as manual	text input	skip /models; missing listing is expected, not degraded

Built-in / import defaults:

• Anthropic, OpenAI, OpenRouter, Ollama → models
• ChatGPT Codex OAuth → static-hint
• Claude Code official api.anthropic.com → models; custom/proxy base → infer-only
• Gemini CLI / OpenCode official maps → models
• Codex config import → infer-only
• Custom add form: discovered list → models; listing failed → infer-only; user chose manual → manual

Independent of sibling PRs #419 / #420 / #421; additive types only.

Type of change

• New feature

Linked issue

Fixes #210

Checklist

• I checked the linked issue / relevant context before starting
• pnpm lint && pnpm typecheck && pnpm test passes locally
• Added/updated tests for the change
• Added a changeset (pnpm changeset) if user-visible
• Updated i18n strings for Settings picker + diagnostics copy

Principles 5b

• Compatible: existing supportsModelsEndpoint / modelsHint still work; modelDiscoveryMode is optional on disk and older configs derive a mode
• Upgradeable: schemaVersion unchanged; Zod accepts the new optional capability field and locksteps it with listing
• Lean: no new dependencies; one shared helper module consumed by IPC, Settings, imports, and diagnostics
• Elegant: listing, picker UX, connection tests, and 404 wording all key off the same mode instead of per-provider fallbacks

Tests

Cover mode helpers, list-for-provider plans, infer-only/manual connection tests (skip /models, success is compatible), Settings picker state, parser payloads, builtin/import stamps, and diagnostics that treat listing 404 as info when the mode does not expect /models.

Screenshots / recordings (UI changes)

Settings picker and custom-provider test copy changed. This environment cannot drive the Electron Settings UI, so those flows were verified with unit tests rather than a live click-through.