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.

docs/manual/07-customization.md

@@ -54,6 +54,25 @@ Example: to increase the scrollback history limit:
     set -gx SCROLLBACK_HISTORY_MAX_FILES 200
 
 
+## Agent Memory Vault
+
+    __fish_agent_vault_dir
+
+    Overrides the agent memory vault location. Defaults to
+    $XDG_DATA_HOME/agent-vault (or ~/.local/share/agent-vault).
+
+    __fish_agent_vault_autopush
+
+    When set to 1, agents-vault also pushes on wrapper launch. Defaults to
+    off: the vault commits locally on every launch and pushes from the
+    Claude Code SessionEnd hook or an explicit agents-vault --push.
+
+    NOTE:
+    With autopush off and no SessionEnd hook installed, backups accumulate
+    locally and never reach the remote. Run agents-vault --status to check
+    how far ahead the vault is.
+
+
 ## Fish Universal Variables
 
 Some settings (fzf colors, theme) are stored in fish_variables via