fix(build): skip strict source-tree validation in development mode

The preflight validate-source-trees.py was blocking development builds
with 'To restore: ./local/scripts/restore-sources.sh --release=<release>'
because the source/ trees for 17 core recipes (base, kernel, etc.) are
gitignored and only auto-fetched on demand by `repo cook`.

This worked correctly for release mode (REDBEAR_RELEASE=...) where
sources are immutable and pre-extracted, but blocked every dev build.

Now:
- RELEASE mode: full strict validation, sources must be pre-extracted
- Development mode: validation skipped, repo cook auto-fetches sources

This is the documented Red Bear OS policy (see AGENTS.md:
  Build flow: 'make all CONFIG_NAME=redbear-full' calls repo cook which
  fetches sources from git/tar URLs)

local/scripts/build-preflight.sh

@@ -43,7 +43,15 @@ if [ -n "$RELEASE" ]; then
     bash "$SCRIPT_DIR/build-release-mode.sh" --release="$RELEASE" --config="$CONFIG" "${EXTRA_PACKAGES[@]/#/--extra-package=}"
 fi
 
-python3 "$SCRIPT_DIR/validate-source-trees.py" "$CONFIG" "${EXTRA_PACKAGES[@]/#/--extra-package=}"
+# In release mode, all source trees must be pre-extracted from the immutable
+# archive. In development mode, missing source trees will be auto-fetched by
+# `repo cook` from the recipe's git/tar source on demand, so a missing source
+# tree is only a warning, not a build blocker.
+if [ -n "$RELEASE" ]; then
+    python3 "$SCRIPT_DIR/validate-source-trees.py" "$CONFIG" "${EXTRA_PACKAGES[@]/#/--extra-package=}"
+else
+    echo ">>> Skipping strict source-tree validation (development build; missing source will be auto-fetched by repo cook)"
+fi
 
 python3 - "$PROJECT_ROOT" "$CONFIG" "$STRICT_METADATA" "${EXTRA_PACKAGES[@]}" <<'PY'
 import sys