chore(ci): Disable GitHub-hosted runner jobs (billing limit)

- external-sentinel: if: false
- telegram-connectivity: if: false

Reason: GitHub account payment/spending limit restrictions
Only self-hosted runner jobs remain active

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/runner-healthcheck.yml

@@ -28,11 +28,12 @@ env:
 
 jobs:
   # ===========================================
-  # 外部哨兵 - 在 GitHub 託管 runner 執行
-  # 即使 110 當機也能發送告警 (核心設計)
+  # 外部哨兵 - 已停用 (2026-03-25)
+  # 原因: GitHub Billing 限制，ubuntu-latest 無法使用
   # ===========================================
   external-sentinel:
     name: "External Sentinel (GitHub-hosted)"
+    if: false  # 🔴 已停用 - GitHub Billing 限制
     runs-on: ubuntu-latest
     timeout-minutes: 5
 
@@ -141,11 +142,12 @@ jobs:
           echo "  - 線上: ${{ steps.api_check.outputs.online_count }} / ${{ steps.api_check.outputs.total_runners }}"
 
   # ===========================================
-  # Telegram 連通性監控
-  # 從 GitHub-hosted runner 監控 188 的 Telegram 連通性
+  # Telegram 連通性監控 - 已停用 (2026-03-25)
+  # 原因: GitHub Billing 限制，ubuntu-latest 無法使用
   # ===========================================
   telegram-connectivity:
     name: "Telegram Connectivity Check"
+    if: false  # 🔴 已停用 - GitHub Billing 限制
     runs-on: ubuntu-latest
     timeout-minutes: 3