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-alias-name-uniqueness.test.ts

@@ -52,7 +52,18 @@ beforeAll(() => {
     'SKILL_PREFIX=0',
     'QUIET=1',
     '_WINDOWS_COPY_NOTE_PRINTED=1',
+    '_FOREIGN_SKIPPED_ENTRIES=()',
+    `SOURCE_GSTACK_DIR="${ROOT}"`,
     extractFn('_link_or_copy'),
+    extractFn('_gstack_link_target_abs'),
+    extractFn('_gstack_target_is_ours'),
+    extractFn('_gstack_generated_header'),
+    extractFn('_claude_entry_is_ours'),
+    extractFn('_claude_entry_owned_strongly'),
+    extractFn('_backup_skill_md'),
+    '_BACKED_UP_SKILL_MDS=()',
+    `_SKILL_BACKUP_ROOT="${os.tmpdir()}/gstack-alias-test-backups"`,
+    extractFn('_write_owned_marker'),
     extractFn('_print_windows_copy_note_once'),
     extractFn('_link_skill_runtime_assets'),
     extractFn('link_claude_skill_dirs'),
@@ -144,7 +155,15 @@ describe('alias installs are rewritten copies (#2511, #2201)', () => {
       const script = [
         'set -e',
         'IS_WINDOWS=0',
+        '_FOREIGN_SKIPPED_ENTRIES=()',
+        `SOURCE_GSTACK_DIR="${ROOT}"`,
         extractFn('_link_or_copy'),
+        extractFn('_gstack_link_target_abs'),
+        extractFn('_gstack_target_is_ours'),
+        extractFn('_gstack_generated_header'),
+        extractFn('_claude_entry_is_ours'),
+        extractFn('_claude_entry_owned_strongly'),
+        extractFn('_write_owned_marker'),
         extractFn('_install_alias_skill_md'),
         extractFn('link_claude_root_skill_alias'),
         `link_claude_root_skill_alias "${ROOT}" "${legacyDir}"`,