Add request-level timeout around provider calls in runQuery

[NeckBeardPrince]

Problem

Provider calls in runQuery (main.go) use a background context with no deadline. A stuck upstream — hung ollama serve, slow/lost TCP connection to api.anthropic.com or api.openai.com, LM Studio server mid-reload — will freeze the CLI until the user sends Ctrl-C. There's no user-visible progress, no error, no recovery.

This was surfaced by Copilot on PR #72: TestProviderRespectsContextCancellation verifies the Provider contract (implementations honor ctx.Done()), but nothing in the app actually imposes a deadline on live provider calls.

Proposal

Wrap the provider call in runQuery with context.WithTimeout:

queryCtx, cancel := context.WithTimeout(ctx, config.RequestTimeout)
defer cancel()
resp, err := provider.Query(queryCtx, systemPrompt, userQuery)

Design notes:

• Default: something generous but finite, e.g. 60s. Streaming responses from Anthropic/OpenAI for short prompts typically complete in a few seconds; local Ollama/LM Studio vary by model size.
• Configurable via:
  • Env var: HOWTFDOI_REQUEST_TIMEOUT (accepts Go duration strings like 30s, 2m)
  • Config file: request_timeout: 60s
• 0 or unset = use default. A negative value could mean "no timeout" for users who genuinely want unbounded calls on slow local models.
• On deadline: return a friendly error (Error: request timed out after 60s. If you're on a slow local model, set HOWTFDOI_REQUEST_TIMEOUT to a larger value.) rather than bubbling the raw context.DeadlineExceeded.

Test plan

• Unit test in main_test.go that wires a blockingMockProvider through runQuery with a tiny timeout and asserts the friendly error message.
• Manual: HOWTFDOI_REQUEST_TIMEOUT=100ms HOWTFDOI_AI_PROVIDER=ollama LMSTUDIO_BASE_URL=http://localhost:1/v1 ./howtfdoi list files should return the timeout error promptly instead of hanging.

Out of scope

• Per-provider timeouts (one value covers all providers; revisit if local models need materially different defaults).
• Retry logic / circuit breaker.