feat(agents-vault): scaffold the vault and link project memory

Creates the vault repo on demand, reusing the AGENTS version bumper and
hook shims, then links the current project's live memory directory into
its slug-keyed entry and commits.

Because the live directory becomes a symlink into the vault, backup and
restore are the same operation: a cloned vault relinks itself on the next
run in each project, with no manifest and no batch restore step.

Also fixes _agents_repo_install_tools' progress messages, which hardcoded
the literal "AGENTS/.agents-tools/" even for callers writing elsewhere:
they now name repo_dir's own basename, so agents-vault reports its own
directory instead of a false AGENTS/ path.

functions/_agents_vault_dir.fish

@@ -0,0 +1,32 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# SYNOPSIS
+#   _agents_vault_dir
+#
+# DESCRIPTION
+#   Prints the agent memory vault root. Honors the universal variable
+#   __fish_agent_vault_dir when set, otherwise
+#   ${XDG_DATA_HOME:-$HOME/.local/share}/agent-vault.
+#
+#   The vault holds agy state as well as Claude state, so it is not nested
+#   under either tool's directory; it is backed-up state rather than
+#   configuration, hence XDG_DATA_HOME rather than XDG_CONFIG_HOME.
+#
+# EXIT STATUS
+#   0  Always
+#
+# RETURNS
+#   The vault root path, one line on stdout.
+#
+# EXAMPLE
+#   set -l vault (_agents_vault_dir)
+function _agents_vault_dir
+    if set -q __fish_agent_vault_dir; and test -n "$__fish_agent_vault_dir"
+        printf '%s\n' "$__fish_agent_vault_dir"
+        return 0
+    end
+    set -l base $XDG_DATA_HOME
+    test -n "$base"; or set base "$HOME/.local/share"
+    printf '%s\n' "$base/agent-vault"
+end