[Feat] Configure Statuspage incident feed by URL

[mrubens]

What changed

• Replace the Cloud-only Statuspage enablement marker and hardcoded feed with the optional R_STATUSPAGE_INCIDENTS_URL setting.
• Fetch incidents from the configured Statuspage-compatible unresolved-incidents feed while preserving the existing cache and fallback behavior.
• Document the variable and cover enabled, disabled, blank, and configured-fetch behavior.

Why

The incident feed location is deployment configuration, not an application constant. Leaving the variable unset now cleanly disables incident banners and Slack warnings.

Impact

Managed deployments must set R_STATUSPAGE_INCIDENTS_URL to retain the feature. The retired marker no longer enables it.

Companion managed-fleet propagation: https://github.com/RooCodeInc/Roomote-Cloud/pull/183

Validation

• mise exec -- pnpm lint
• mise exec -- pnpm check-types
• mise exec -- pnpm --filter @roomote/docs check
• mise exec -- pnpm --filter @roomote/env test -- --run src/__tests__/index.test.ts
• mise exec -- pnpm --filter @roomote/slack test -- --run src/__tests__/statuspage-incidents.test.ts
• Repository pre-push checks

[roomote-community]

No new code issues found. See task

• Scope Statuspage Redis cache, negative-cache, and lock keys by the configured feed URL (packages/slack/src/statuspage-incidents.ts:118).

_{Reviewed 3d9e2fb}