fix: send opencode User-Agent on upstream requests to avoid Zen free-tier 429

[cheerc]

What

Upstream HTTP requests from the opencode-zen and opencode-go providers now carry a User-Agent: opencode/routatic-proxy header. Users can route OpenCode Zen free models (e.g. deepseek-v4-flash-free) through the proxy without hitting 429 FreeUsageLimitError → 502 all models failed.

Why

Closes #137. opencode.ai's Zen free tier rate-limits by client identity: requests whose User-Agent starts with opencode/ are exempt; anything else (Go's default Go-http-client/1.1, curl) gets HTTP 429 {"type":"FreeUsageLimitError","message":"Rate limit exceeded. Please try again later."}. The proxy never set a User-Agent on upstream requests, so all Zen free-model traffic was throttled even though the same account key works fine from the native opencode CLI. CONFIGURATION.md/MODELS.md advertise Zen free models as supported fallbacks, so this is a documented-behavior violation, not a usage issue.

How

• Added one shared const upstreamUserAgent = "opencode/routatic-proxy" in internal/provider/opencode_zen.go (the opencode/ prefix is what the free-tier check keys on).
• Set User-Agent alongside the existing headers at all upstream request construction sites: Zen doRequest (chat completions, stream + non-stream), doJSONRequest (Responses/Gemini), executeAnthropic, streamAnthropic; Go doRequest, executeAnthropic, streamAnthropic.
• Tests drive the real providers through httptest upstream servers asserting the opencode/ prefix (and existing auth headers) for every wire format / stream variant.

Scope

• Included: User-Agent on the 7 upstream request sites across opencode_zen.go / opencode_go.go + regression tests in internal/provider/opencode_useragent_test.go.
• Excluded: the internal/client package (used by the router's Claude/Anthropic and OpenRouter paths) — not implicated in the Zen free-tier bug; the auth-error short-circuit behavior in [bug] Do not fall back when encountering a usage limit #101 and [bug] Auth errors (401/403) should short-circuit the fallback chain #108 is untouched.
• Deviations: none — follows the reproduction in [bug] Zen free models 429 FreeUsageLimitError because upstream requests use generic Go User-Agent #137 exactly.

Verification

• RED (fix absent): go test ./internal/provider/ -run OpencodeUserAgent -count=1 → 9/9 FAIL, all with User-Agent = "Go-http-client/1.1", want prefix "opencode/".
• GREEN (fix present): same command → ok 9/9 PASS.
• Regression: go test ./... -count=1 → all packages ok.
• gofmt -l internal/provider/ → clean; go vet ./internal/provider/ → ok.
• Manual (local binary built from this change): proxy-served deepseek-v4-flash-free returns 200; same request was 429/502 before.
• CI: pending.

Current HEAD: 229fec4

Risks

None known. The header is cosmetic to upstream; no behavior change for users who already use paid opencode-go / Claude / OpenRouter paths. If the upstream free-tier check ever changes, tests at internal/provider/opencode_useragent_test.go will flag it.

Lessons

The free-tier limiter keys on the effective User-Agent of the upstream client, not on the API key alone. Any future proxy client should mirror the first-party client's User-Agent to stay in the same quota bucket.

Closes #137