feat(ai): add a requesty provider alias

[Thibaultjaigu]

What and why

Adds Requesty (an OpenAI compatible LLM gateway) as a PGBOT_AI_PROVIDER alias for explain and ask, wired the same way as the existing openrouter alias. No new client code: it reuses the /chat/completions provider and only picks the endpoint and key variable.

Changes:

• internal/ai/openai.go: defaultRequestyURL = "https://router.requesty.ai/v1" next to defaultOpenRouterURL.
• internal/ai/resolve.go: requesty joins the /chat/completions case, maps to its endpoint, and reads its key from REQUESTY_API_KEY (or PGBOT_AI_API_KEY) only, so an OpenAI or OpenRouter key in the shell is never sent to it. keyVarsFor names REQUESTY_API_KEY in the missing key error. It is not added to auto detection, so it only runs when selected explicitly.
• internal/ai/resolve_test.go: covers the endpoint with both key variables, the refusal to borrow OPENAI_API_KEY, and that REQUESTY_API_KEY alone is not auto detected.
• README environment table and explain section (including the EU endpoint https://router.eu.requesty.ai/v1 via PGBOT_AI_BASE_URL), plus a changelog entry under Unreleased.

How to test:

export PGBOT_AI_PROVIDER=requesty
export REQUESTY_API_KEY=rqsty-...
export PGBOT_AI_MODEL=openai/gpt-4o-mini   # optional, default gpt-5.6-terra also works
pgbot ask "What needs attention?" "$DATABASE_URL"

Validation:

• scripts/gate.sh passes on HEAD (build, vet, golangci-lint 0 issues, tests, 6 targets). gofmt -l clean.
• Live call through Resolve() and openaiModel.Generate against Requesty: HTTP 200 with openai/gpt-4o-mini, and HTTP 200 with the default gpt-5.6-terra through the reasoning path (max_completion_tokens and reasoning_effort).
• govulncheck not run locally; no dependencies changed.

Checklist

• scripts/gate.sh passes (builds HEAD, not just the working tree)
• New SQL is read-only; no EXPLAIN ANALYZE; findings stay deterministic (computed in Go) (no SQL touched)
• No PII enters a model.Context / --json / the store (unchanged)
• --json change is additive, or model.SchemaVersion bumped + schema regenerated (go run ./tools/schemagen) (no --json change)
• A new finding has a docs/findings/<id>.md page + catalog entry (no new finding)

Disclosure: I work at Requesty. Happy to adjust anything to match project conventions.