docs: add missing external-services guide

[Frankie-Xu]

Summary

• Add docs/external-services.md, the dependency matrix already linked from README Quick Start and Additional Guides (those links currently 404).
• Rebased onto current main (includes feat(retrieval): default to agent explore and retire map-nav #395). Env keys rechecked against the latest apps/api/.env.example and apps/worker/.env.example, including agent-explore keys (AGENT_EXPLORE_HARNESS, CURSOR_API_KEY, AGENT_EXPLORE_CURSOR_MODEL).
• HTML / HTM support status is not changed here. main already lists .html .htm as supported (README rewrite around 2026-09-10). This PR only fills the missing guide.

Closes #333 (remaining half: missing file; format-list half already on main).

Verification

• Every env key in the new guide appears in .env.example (API and/or worker). No invented keys. No real credentials or private endpoints.
• README docs/ relative links all resolve, including the newly added docs/external-services.md.
• uvx ruff check apps packages — All checks passed. Docs-only; no API/worker/runtime change.

Deployment Notes

• No new or changed environment variables.
• No database migrations, queue changes, storage changes, or release-order requirements.
• Backwards compatible; rollback is reverting this PR.

Checklist

• Tests were added or updated when behavior changed — N/A (docs-only)
• Public docs, examples, or OpenAPI contracts were updated when needed
• Database migrations are idempotent and safe to deploy — N/A
• Logs, errors, and validation paths avoid leaking secrets or user data
• The pull request description explains any breaking or user-visible change — none; README format list unchanged