fix: expand model key regex and fallbackOn categories

[tmogeid]

Problem

The MODEL_KEY_RE regex in src/config/schema.ts and src/config/agent-loader.ts uses charset [a-zA-Z0-9._-] for the modelID portion, which rejects model keys containing forward slashes or colons.

This blocks users from configuring fallback models with nested provider paths (e.g. nvidia/z-ai/glm-5.2, openrouter/tencent/hy3:free).

Additionally, the ErrorCategory type only supports 5 categories (rate_limit, quota_exceeded, 5xx, timeout, overloaded), which doesn't cover common failure modes like auth errors, billing issues, or premium subscription requirements.

Changes

1. Fix MODEL_KEY_RE regex

Add \/: to the modelID charset in both regex definitions:

- /^[a-zA-Z0-9_-]{1,100}\/[a-zA-Z0-9._-]{1,100}$/
+ /^[a-zA-Z0-9_-]{1,100}\/[a-zA-Z0-9._\/:-]{1,100}$/

This allows nested provider paths with slashes and colons (for model IDs like openrouter/tencent/hy3:free).

2. Expand ErrorCategory to 29 categories

The original 5 categories only covered rate limits, quotas, server errors, timeouts, and overload. The expanded set adds:

• Auth errors: unauthorized, 401, 403
• Credit errors: insufficient_credits, insufficient_balance, no_credits, out_of_credits, insufficient_funds
• Billing errors: billing, credit_balance, payment_required
• Premium barriers: premium, premium_subscription, subscription_required
• HTTP status codes: 429, 500, 502, 503, 504
• Limit errors: limit, insufficient_quota, quota_exceeded_message

3. Update classifier with new patterns

The classifier now recognizes messages like "unauthorized", "insufficient balance", "premium subscription", etc. and maps them to the appropriate categories.

4. Fix README

• opencode.jsonc -> opencode.json (the correct filename)
• Added nested provider path examples
• Added extended fallbackOn categories documentation

Testing

• TypeScript typecheck passes (tsc --noEmit — 0 errors)
• All detection/config/agent-loader tests pass (51/51)
• Full suite: 166 pass, 2 pre-existing failures (Windows file I/O, unrelated)