ci: add Gitea Actions CI/CD, branch workflow, and contributor docs

- .gitea/workflows/ci.yml: build + test KisaniCal scheme on a self-hosted
  macOS runner for every PR into main/develop (and pushes to develop).
- .gitea/workflows/release.yml: archive + export IPA on push to main, with
  a commented TestFlight upload placeholder (App Store Connect API key).
- ExportOptions.plist: export template (Team ID K8BLMMR883, app-store method).
- scripts/gitea-setup.sh: idempotent Gitea API setup for develop default
  branch + main branch protection.
- CONTRIBUTING.md: feature/* -> develop -> main workflow and PR gate rules.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

ExportOptions.plist

@@ -0,0 +1,45 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  ExportOptions template for `xcodebuild -exportArchive`.
+
+  FILL IN:
+    • teamID          — your Apple Developer Team ID. Detected from project.yml
+                        (DEVELOPMENT_TEAM): K8BLMMR883  ← pre-filled below, verify it.
+    • method          — choose ONE:
+                          app-store   → TestFlight / App Store submission
+                          ad-hoc      → distribute to registered UDIDs
+                          development  → internal dev devices
+                          enterprise   → in-house (Apple Enterprise Program only)
+
+  Notes:
+    • signingStyle "automatic" lets Xcode manage certs/profiles (pairs with
+      -allowProvisioningUpdates in release.yml). Switch to "manual" if you
+      install a specific distribution profile on the runner.
+-->
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+    <key>method</key>
+    <string>app-store</string>
+
+    <key>teamID</key>
+    <string>K8BLMMR883</string>
+
+    <key>signingStyle</key>
+    <string>automatic</string>
+
+    <key>uploadSymbols</key>
+    <true/>
+
+    <key>uploadBitcode</key>
+    <false/>
+
+    <!-- Optional: pin the app to a specific provisioning profile (manual signing).
+    <key>provisioningProfiles</key>
+    <dict>
+        <key>com.kutesir.KisaniCal</key>
+        <string>YOUR_PROFILE_NAME</string>
+    </dict>
+    -->
+</dict>
+</plist>