Add a Notion service configuration

[shreyanshjain7174]

What

Add Notion as a registered service, following the exact pattern already used by github, slack, stripe, and google_workspace.

Why

More service configurations mean more agents can use AgentGate out of the box without writing any Go code — this is a config-only, no-code-required contribution.

Files to touch

• configs/services.yaml — the file the Dockerfile and quickstart actually load. Add a notion: entry.
• config/services.yaml — the local, non-Docker dev default. Add the same entry for consistency (see internal/registry/production_config_test.go's TestProductionConfig_LocalDevDefaultMatches, which checks these two files don't drift).
• configs/services/notion.yaml — a new per-service reference file, matching the convention README.md points readers at (see configs/services/google_workspace.yaml for the exact shape to copy).

Suggested shape

Notion's API (https://developers.notion.com/reference/intro) uses Bearer token auth (an internal integration secret or an OAuth token) and a fixed Notion-Version header is normally required — AgentGate's current injectAuth doesn't send extra static headers per service yet, so start with the two simplest read-only actions that work without one:

notion:
  base_url: https://api.notion.com
  auth:
    type: bearer
  actions:
    search:
      method: POST
      path: /v1/search
      params:
        query: "string?"
    retrieve_page:
      method: GET
      path: /v1/pages/{page_id}
      params:
        page_id: string

(If you find the Notion-Version header is required for a real call, note that in your PR description rather than silently dropping the action — it's fine to file this as a known limitation.)

Acceptance checks

• configs/services.yaml, config/services.yaml, and configs/services/notion.yaml all parse as valid YAML and match the registry.Config schema in internal/registry/registry.go.
• go test ./internal/registry/... passes, including the existing TestProductionConfig_* tests (they check for specific named services being present; adding a new one doesn't break them).
• No Go code changes are required for this issue.

Test path (no secrets needed)

go test ./internal/registry/... -run TestProductionConfig -v

This test loads the real config files directly — no live Notion account, API key, or network access needed.