fix(provider): do not poll LM Studio unless explicitly configured

[jfenal]

Problem

LM Studio was hard-coded to http://127.0.0.1:1234 in startDiscovery and polled every 30 seconds on every install — even on machines where it is not installed or configured. This produced continuous WARN log noise (connection refused on 127.0.0.1:1234) and a constant background network probe.

Fix

cmd/tinycode/config.go: only pass the LM Studio URL to discovery when the user has explicitly opted in, which happens when:

• TINYCODE_LMSTUDIO_HOST is set (existing behavior, unchanged), or
• an lm-studio provider block is present in the config file (new — restores the URL to the standard default http://127.0.0.1:1234).

Otherwise the URL stays empty and Discovery.poll skips the provider entirely (same mechanism vLLM already uses). Ollama is unchanged.

Verification

• go build ./cmd/tinycode — passes
• go vet ./cmd/tinycode/ — passes
• go test ./cmd/tinycode/ + go test ./internal/provider/ — pass

Fixes #28

[bobbyjohnstx]

Closing without merging. The continuous WARN and 30-second probe of an unused LM Studio (127.0.0.1:1234, #28) is handled on main by a different change:

• d17514ea — a provider whose last probe succeeded is checked every 15 minutes. One failure returns it to the 30-second loop; three failures use the existing 2–10 minute dormant backoff. A missing provider is logged at info, not warning. /connect forces a real rescan.
• 8231de02 — notes that this covers the fix(provider): do not poll LM Studio unless explicitly configured #39 / fix(provider): LM Studio should not be polled by default if unconfigured #28 noise while LM Studio stays in the startup scan.

This PR's opt-in (skip LM Studio unless TINYCODE_LMSTUDIO_HOST or an lm-studio config block is set) is not what we want: startup should still try LM Studio along with Ollama, vLLM, and OpenRouter.