Add agent instructions; extract signing/backend secrets from source

- Add AGENTS.md (canonical) + CLAUDE.md symlink + ROADMAP.md
- Move Apple Team ID from project.yml into a gitignored
  Config/Signing.xcconfig via configFiles; commit the .example template
- Replace hardcoded backend host in AppSettings with a neutral
  placeholder + SPARK_BACKEND_URL env-var fallback
- Scrub the Team ID, .local host, and raw LAN IP from README/docs
- Ignore Config/Signing.xcconfig and .env

.gitignore

@@ -17,3 +17,10 @@ build/
 
 # Personal call screenshots / fixtures (faces, contact names) — never commit
 example-screenshots/
+
+# Local signing identity (Apple Team ID) — keep out of source; template is committed
+Config/Signing.xcconfig
+
+# Local env files (e.g. SPARK_BACKEND_URL for dev/harness runs) — never commit
+.env
+.env.local