fix(agents-vault): dedupe local-slug formula, drop dead code, widen migration coverage

The slug-migration fallback (used when there is no live symlink to read
the previous slug from) recomputed the local-* candidate by lowercasing
the basename only, while _agents_repo_slug sanitizes it. The two formulas
had drifted, so the fallback silently found nothing for any project
directory whose basename needed sanitizing.

Extract the formula into a single private helper,
_agents_repo_local_slug, and have both _agents_repo_slug's no-remote
branch and agents-vault's migration fallback call it, so there is one
place left to drift.

Also drop two dead lines the review flagged: an unused  local,
and an unreachable mkdir -p (path dirname ...) — slugs never contain a
path separator, so dirname always resolves to a directory that already
exists by that point.

Widen migration test coverage: the current-entry-present-but-empty case,
a remote URL rewrite, a remote removal, and a dirty-basename fallback
test that fails without the sanitization fix and passes with it.

functions/_agents_repo_local_slug.fish

@@ -0,0 +1,40 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# SYNOPSIS
+#   _agents_repo_local_slug <dir>
+#
+# DESCRIPTION
+#   Builds the path-derived fallback slug used when a project has no git
+#   remote: local-<sanitized-basename>-<8 hex of sha256(realpath)>. The
+#   basename is lowercased and every character outside [a-z0-9._-] is
+#   mapped to a dash, matching the sanitization the remote-URL branch of
+#   _agents_repo_slug applies to hostnames and paths.
+#
+#   This is the single source of truth for that formula. It exists so the
+#   rule is written once: _agents_repo_slug's no-remote branch calls it to
+#   produce the slug, and agents-vault's slug-migration fallback (used when
+#   there is no live symlink yet to read the previous slug from) calls it
+#   to recompute the same candidate. Duplicating the formula in both places
+#   let them drift once before; this closes that gap for good.
+#
+# ARGUMENTS
+#   dir  Absolute or relative path to the project directory
+#
+# EXIT STATUS
+#   0  Slug printed
+#   1  No directory argument given
+#
+# RETURNS
+#   The local-* slug, one line on stdout.
+#
+# EXAMPLE
+#   set -l slug (_agents_repo_local_slug /home/user/myproject)
+function _agents_repo_local_slug --argument-names dir
+    test -n "$dir"; or return 1
+
+    set -l rp (path resolve "$dir")
+    set -l base (string lower -- (path basename "$rp") | string replace -ra '[^a-z0-9._-]' '-')
+    set -l digest (printf '%s' "$rp" | sha256sum | string split -f1 ' ')
+    printf 'local-%s-%s\n' "$base" (string sub -l 8 -- "$digest")
+end

functions/_agents_repo_slug.fish

@@ -1,6 +1,9 @@
 # Copyright (C) 2026 Rootiest
 # SPDX-License-Identifier: AGPL-3.0-or-later
 
+# DEPENDENCIES
+#   _agents_repo_local_slug
+#
 # SYNOPSIS
 #   _agents_repo_slug <dir>
 #
@@ -63,8 +66,5 @@ function _agents_repo_slug --argument-names dir
         return 0
     end
 
-    set -l rp (path resolve "$dir")
-    set -l base (string lower -- (path basename "$rp") | string replace -ra '[^a-z0-9._-]' '-')
-    set -l digest (printf '%s' "$rp" | sha256sum | string split -f1 ' ')
-    printf 'local-%s-%s\n' "$base" (string sub -l 8 -- "$digest")
+    _agents_repo_local_slug "$dir"
 end

functions/agents-vault.fish

@@ -5,8 +5,9 @@
 #   12-ai-and-developer-tools
 #
 # DEPENDENCIES
-#   _agents_vault_dir, _agents_repo_slug, _agents_repo_ensure_symlink,
-#   _agents_repo_sync, _agents_repo_install_tools, git, hostname
+#   _agents_vault_dir, _agents_repo_slug, _agents_repo_local_slug,
+#   _agents_repo_ensure_symlink, _agents_repo_sync,
+#   _agents_repo_install_tools, git, hostname
 #
 # SYNOPSIS
 #   agents-vault [--link] [--push] [--restore] [--status]
@@ -230,17 +231,13 @@ function agents-vault --description 'track curated agent memory in a host-scoped
         end
         if test -z "$prev_slug"
             # No link yet (fresh machine): try the path-derived candidate.
-            set -l cand_rp (path resolve "$root")
-            set -l cand_base (string lower -- (path basename "$cand_rp"))
-            set -l cand_digest (printf '%s' "$cand_rp" | sha256sum | string split -f1 ' ')
-            set -l cand "local-$cand_base-"(string sub -l 8 -- "$cand_digest")
+            set -l cand (_agents_repo_local_slug "$root")
             if test "$cand" != "$slug"; and test -d "$vault/projects/$cand"
                 set prev_slug $cand
             end
         end
 
         if test -n "$prev_slug"; and test "$prev_slug" != "$slug"
-            set -l prev_mem "$vault/projects/$prev_slug/claude/memory"
             set -l cur_content
             test -d "$vmem"; and set cur_content (command ls -A "$vmem" 2>/dev/null)
             if test (count $cur_content) -gt 0
@@ -249,7 +246,6 @@ function agents-vault --description 'track curated agent memory in a host-scoped
                 return 1
             end
             test -d "$vmem"; and rm -rf "$vault/projects/$slug"
-            mkdir -p (path dirname "$vault/projects/$slug")
             if not git -C "$vault" mv "projects/$prev_slug" "projects/$slug" 2>/dev/null
                 command mv "$vault/projects/$prev_slug" "$vault/projects/$slug"; or return 1
             end