fix(setup,relink): ownership proof has two strengths; weak proof never deletes a directory or discards a differing file

The first #2119 gate treated a byte-identical or banner-bearing real-file
SKILL.md as full ownership, so a prefix flip could rm -rf a user's directory
(their own qa skill started from a gstack SKILL.md, plus my-templates/) and
the link pass could replace their customized file with a symlink. Two
strengths now:

- STRONG: the .gstack-owned marker (we created the directory), or a
  directory holding nothing but symlinks and the marker (deleting it loses
  no data). Only strong proof removes a directory whole.
- WEAK: byte-identity with our source or the two-line gen-skill-docs banner
  on a real file. Weak proof covers that SKILL.md and our runtime-asset
  links only; a differing file is moved to
  ${GSTACK_HOME:-~/.gstack}/backups/skills/<ts>/<skill>/ before we link
  over it, and setup/relink print one summary line naming what moved.

The marker is written on every platform now (path-independent proof for
Windows copies and for checkouts whose path carries no gstack segment), but
only for a directory gstack creates: a directory we merely link into
(unclaimed, or a legacy install) never becomes deletable whole. A directory
with no SKILL.md at all is unclaimed: the link pass may add our file, the
cleanup pass has nothing to remove.

Also from the review passes: the banner check reads 8192 bytes, not 40
lines (investigate, office-hours, plan-ceo-review and design-consultation
carry the banner past line 40 and were left "foreign" on pre-marker
Windows installs); a link into a checkout named without a gstack segment
(git worktree add ../gstack-<branch>) is ours when that tree carries
setup + VERSION + bin/; relink's fast path is gone so both files
canonicalize before judging; relink's root alias (_gstack-command) is
gated and stamped like every other entry; relink reports the bare entry
name with setup's wording and setup dedupes when forwarding
(_run_relink_quiet); the summary names the browser skills as examples.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

test/setup-cleanup-orphans.test.ts

@@ -24,7 +24,7 @@ function extractFn(name: string): string {
 }
 
 function cleanupBody(): string {
-  return extractFn('_gstack_generated_header') + extractFn('cleanup_old_claude_symlinks');
+  return [extractFn('_gstack_link_target_abs'), extractFn('_gstack_target_is_ours'), extractFn('_gstack_dir_only_links'), extractFn('_cleanup_linked_dir'), extractFn('_gstack_generated_header'), extractFn('_cleanup_weak_dir'), extractFn('cleanup_old_claude_symlinks')].join('\n');
 }
 
 describe('setup: cleanup_old_claude_symlinks — static (#2204)', () => {
@@ -68,7 +68,7 @@ describe.skipIf(process.platform === 'win32')('setup: cleanup_old_claude_symlink
     const script = [
       'set -e',
       `IS_WINDOWS=${opts.isWindows ?? '0'}`,
-      extractFn('_gstack_generated_header'),
+      extractFn('_gstack_link_target_abs'), extractFn('_gstack_target_is_ours'), extractFn('_gstack_dir_only_links'), extractFn('_cleanup_linked_dir'), extractFn('_gstack_generated_header'), extractFn('_cleanup_weak_dir'),
       extractFn('cleanup_old_claude_symlinks'),
       `cleanup_old_claude_symlinks "${gstackArg}" "${skills}"`,
     ].join('\n');