feat: add maintenanceWindows resource for scheduling planned downtime

[caballeto]

Summary

Adds a client.maintenanceWindows resource module so users can schedule
planned alert-suppression windows directly from the SDK — mirroring the
/api/v1/maintenance-windows surface that the API and dashboard already
expose. Closes the gap with cli / sdk-python, which previously had no
JS/TS counterpart for this workflow.

The resource is a thin, fully typed wrapper over the existing OpenAPI
contract — no new generated types, no spec changes, no runtime deps.

Use case

import {Devhelm} from '@devhelm/sdk'

const client = new Devhelm({token: process.env.DEVHELM_API_TOKEN!})

// Schedule a window for the upcoming DB migration
const window = await client.maintenanceWindows.create({
  monitorId: process.env.DB_MONITOR_ID,
  startsAt: '2026-06-01T03:00:00Z',
  endsAt:   '2026-06-01T04:00:00Z',
  reason:   'Quarterly db migration',
})

// What's currently in progress org-wide?
const active = await client.maintenanceWindows.list({filter: 'active'})

// Pull the plug if we finish early
await client.maintenanceWindows.cancel(window.id)

What's included

• src/resources/maintenance-windows.ts — MaintenanceWindows class with
  create, list (auto-paginating), listPage, get, update, cancel.
  Each method is async, JSDoc-documented with a @example, and returns the
  generated MaintenanceWindowDto (or Page<MaintenanceWindowDto>), never
  any.
• Bridge updates in src/schemas.ts and src/types.ts for the
  MaintenanceWindowDto, CreateMaintenanceWindowRequest,
  UpdateMaintenanceWindowRequest, plus the SingleValueResponse /
  TableValueResult envelopes.
• Wiring in src/client.ts and the MaintenanceWindows /
  MaintenanceWindowFilters re-exports in src/index.ts.
• Five new entries in test/spec-paths.test.ts so spec drift around
  /api/v1/maintenance-windows is caught at test time.
• test/maintenance-windows.test.ts — stubs globalThis.fetch (same shape
  as the existing surface-telemetry tests) and asserts the exact path,
  method, query string, and body for every method, plus auto-pagination
  and client-side validation behavior.

Test plan

• npm run lint — clean
• npm run typecheck — clean (strict mode, no any)
• npm run build — succeeds
• npm test — all 1321 tests pass (including 13 new
  maintenance-windows.test.ts cases)
• CI green on the PR

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