skeleton: bring AppImage parity, scaffolded apps inherit the packaging flow

The v0.1.0 shakedown fixes for AppImage assembly (path-repo
symlink:false sed, writable-cache redirect) all landed in
examples/todo. The skeleton — which is what bin/php-qml-init copies
when scaffolding a new app — had no `appimage` target at all, so every
scaffolded app would have to either copy the example's Makefile by
hand or re-discover the same shakedown bugs.

Brings parity:

  - framework/skeleton/Makefile gains `staging-symfony` and `appimage`
    targets, mirroring the example's. Two new variables (BUNDLE_SRC,
    PACKAGING) parameterise the framework-tree paths so bin/php-qml-init
    can rewrite them at scaffold time without sed-touching the recipe.
  - framework/skeleton/packaging/skeleton.{desktop,png} added — minimum
    surface for the AppImage assembly to succeed without the user
    needing to author them.
  - framework/skeleton/Makefile's staging-symfony recipe handles both
    relative (framework default `../../php`) and absolute (post-scaffold)
    BUNDLE_SRC values via a case statement.
  - bin/php-qml-init renames packaging/skeleton.* → packaging/$NAME.*,
    rewrites the .desktop file's Name/Exec/Icon, and updates the
    Makefile's --app-name / --output / --desktop / --icon flags +
    BUNDLE_SRC + PACKAGING variables. For --vendor mode, framework's
    packaging/linux/ is also vendored to .bridge-packaging/ alongside
    the existing .bridge/ + .bridge-qml/.

Verified by scaffolding both modes:
  - non-vendored: BUNDLE_SRC + PACKAGING absolute paths
  - --vendor: BUNDLE_SRC=../.bridge, PACKAGING=.bridge-packaging,
    .bridge-packaging/ contains build-appimage.sh

Skeleton's `make quality` still green; staging-symfony works locally
(vendor/php-qml/bridge resolves to a real directory, not a symlink).

Closes the v0.1.1 follow-up "bin/php-qml-init parity" tracked in
PLAN.md §13.

Bundled drive-by: docs/makers.md picked up two markdownlint auto-fixes
(blank lines around lists) when the IDE saved.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

framework/skeleton/packaging/skeleton.desktop

@@ -0,0 +1,8 @@
+[Desktop Entry]
+Type=Application
+Name=php-qml Skeleton
+Comment=php-qml application scaffold (replace with your own)
+Exec=skeleton
+Icon=skeleton
+Categories=Utility;
+Terminal=false