chore: add CLAUDE.md, SPDX-header hook, and preflight skill

- CLAUDE.md: build gotchas (JDK 17-21, AGP built-in Kotlin, KSP-not-KAPT),
  test stack, the SPDX header rule, Conventional Commits, and commands.
- .claude/settings.json + hooks/check-spdx.py: PostToolUse hook that warns
  (non-blocking) when a .kt/.kts file lacks the SPDX license header.
- .claude/skills/preflight: runs the fast CI gate (assembleDebug +
  testDebugUnitTest + lintDebug) locally.

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

.claude/hooks/check-spdx.py

@@ -0,0 +1,61 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-3.0-or-later
+"""PostToolUse (Write|Edit) hook: warn when a Kotlin source file is missing the
+SPDX license header that every LibreMail source file must carry.
+
+Non-blocking: prints a JSON warning (systemMessage + additionalContext so Claude
+adds the header) and always exits 0. Any error is swallowed so the tool flow is
+never broken by this check.
+"""
+import json
+import sys
+
+REQUIRED = "SPDX-License-Identifier"
+HEADER_LINE = "// SPDX-License-Identifier: GPL-3.0-or-later"
+
+
+def main() -> int:
+    try:
+        data = json.load(sys.stdin)
+    except Exception:
+        return 0
+
+    path = (data.get("tool_response") or {}).get("filePath") \
+        or (data.get("tool_input") or {}).get("file_path")
+    if not path:
+        return 0
+
+    lower = path.lower()
+    if not (lower.endswith(".kt") or lower.endswith(".kts")):
+        return 0
+
+    try:
+        with open(path, "r", encoding="utf-8", errors="replace") as fh:
+            content = fh.read()
+    except OSError:
+        # File missing (e.g. a delete) or unreadable — nothing to check.
+        return 0
+
+    if REQUIRED in content:
+        return 0
+
+    msg = f"SPDX header missing in {path}"
+    out = {
+        "systemMessage": msg,
+        "hookSpecificOutput": {
+            "hookEventName": "PostToolUse",
+            "additionalContext": (
+                f'{path} is missing the license header. Add "{HEADER_LINE}" as the '
+                "first line — every LibreMail source file carries it."
+            ),
+        },
+    }
+    print(json.dumps(out))
+    return 0
+
+
+if __name__ == "__main__":
+    try:
+        sys.exit(main())
+    except Exception:
+        sys.exit(0)

.claude/settings.json

@@ -0,0 +1,17 @@
+{
+  "hooks": {
+    "PostToolUse": [
+      {
+        "matcher": "Write|Edit",
+        "hooks": [
+          {
+            "type": "command",
+            "command": "python .claude/hooks/check-spdx.py",
+            "timeout": 30,
+            "statusMessage": "Checking SPDX header"
+          }
+        ]
+      }
+    ]
+  }
+}

.claude/skills/preflight/SKILL.md

@@ -0,0 +1,35 @@
+---
+name: preflight
+description: Run LibreMail's fast CI gate locally (assembleDebug + testDebugUnitTest + lintDebug) before pushing or opening a PR. Mirrors the merge gate; does NOT run emulator E2E. Use before treating a change as done.
+---
+
+# /preflight
+
+Run the same fast checks CI enforces on every PR, in order, and report the outcome.
+
+## Preconditions
+
+- The Gradle daemon must run on **JDK 17–21**. AGP 9.2 fails on JDK 25+. If a build errors
+  with a JDK/AGP version mismatch, check `java -version` / `JAVA_HOME` and point it at a 17–21
+  JDK (e.g. Android Studio's bundled JBR) before retrying.
+- PowerShell: invoke the wrapper as `.\gradlew`. Git Bash / the Bash tool: `./gradlew`.
+
+## Steps
+
+Run these three, stopping at the first failure:
+
+```bash
+./gradlew :app:assembleDebug
+./gradlew :app:testDebugUnitTest
+./gradlew :app:lintDebug
+```
+
+To keep going and collect every failure in one pass, add `--continue`.
+
+## Reporting
+
+- If all three pass, say so plainly (e.g. "preflight green: build, unit tests, lint").
+- On failure, surface the actual Gradle error. For test failures, point at the report under
+  `app/build/reports/tests/testDebugUnitTest/`; for lint, `app/build/reports/lint-results-debug.html`.
+- Do **not** run emulator/E2E (`connectedDebugAndroidTest`) here — that's CI's job unless the
+  user explicitly asks.