fix(mem0-plugin): thread cwd to resolvers, add write-path tests, userConfig for API key

- on_pre_compact.py and capture_compact_summary.py now read cwd from
  hook stdin and pass it to resolve_project_id/resolve_branch (monorepo safe)
- 5 new tests verify all write functions use app_id top-level and never
  put project_id in metadata
- plugin.json declares userConfig.MEM0_API_KEY so Claude Code prompts
  for the key at install time (sensitive, stored in system keychain)

mem0-plugin/scripts/capture_compact_summary.py

@@ -153,9 +153,10 @@ def main():
         return
 
     session_id = hook_input.get("session_id", "")
+    cwd = hook_input.get("cwd") or None
     user_id = resolve_user_id()
-    project_id = resolve_project_id()
-    branch = resolve_branch()
+    project_id = resolve_project_id(cwd)
+    branch = resolve_branch(cwd)
 
     lines = tail_lines(transcript_path, MAX_TAIL_LINES)
     if not lines:

mem0-plugin/scripts/on_pre_compact.py

@@ -234,9 +234,10 @@ def main():
         return
 
     session_id = hook_input.get("session_id", "")
+    cwd = hook_input.get("cwd") or None
     user_id = resolve_user_id()
-    project_id = resolve_project_id()
-    branch = resolve_branch()
+    project_id = resolve_project_id(cwd)
+    branch = resolve_branch(cwd)
 
     lines = tail_lines(transcript_path, MAX_TAIL_LINES)
     if not lines: