Fix second Codex finding: packaging symlink produced a dangling-symlink-only archive

The ensure-app-name-symlink fix from the previous commit created
../extended_occweb as a symlink so the inherited "../$(app_name)" tar
references would resolve regardless of the checkout's real directory
name - but the tar invocations ran without --dereference, so GNU tar
stored the symlink itself rather than following it into the real
files. make source/appstore therefore produced a 1-entry archive
containing nothing but a dangling absolute symlink, unusable on any
other machine. Caught by a second independent Codex pass; my own
"make appstore" test after the first fix only checked the archive's
top-level entry name, not its actual file count, so this went
unnoticed.

Rather than patch this with --dereference, replaced the whole
symlink+raw-tar approach with `git archive --prefix=$(app_name)/
HEAD`: it sets the correct top-level folder name directly (no symlink
needed at all), and - caught during this fix's own verification,
not by either Codex pass - it also closes a separate real leak the
symlink approach had: tarring the checkout directory directly
included every untracked/gitignored file too (e.g.
notes/deploy-nc01.md, which exists specifically because it must
never leave this machine). `git archive` only ever includes tracked
files. Dev/CI-only files (tests/, Makefile, composer.*, phpunit*.xml,
.travis.yml) are now stripped via `export-ignore` entries in a new
.gitattributes instead of a long list of tar --exclude flags repeated
per target - also means source and appstore now produce identical
output, which is fine given neither is part of our actual release
flow yet.

Verified end to end this time: extracted the resulting archive and
confirmed file count, absence of symlinks, absence of tests/notes/
dev files, and correct info.xml content - not just the archive's
top-level listing.

.gitattributes

@@ -0,0 +1,11 @@
+# Files excluded from `git archive` output - used by `make appstore` (and by
+# our own nc01 deploy tarballs) to strip dev/CI-only files that don't belong
+# in a running install, without needing them repeated as tar --exclude flags.
+tests export-ignore
+.travis.yml export-ignore
+phpunit.xml export-ignore
+phpunit.integration.xml export-ignore
+composer.json export-ignore
+composer.lock export-ignore
+Makefile export-ignore
+.gitattributes export-ignore