fix(agents-vault): always link the current project's memory

The guard around the symlink step only linked when the live Claude
project directory already existed, which is exactly backwards for the
clone-onto-a-new-machine restore case: a freshly cloned vault entry
would be silently left unlinked and a starting agent would write fresh,
history-less memory instead. _agents_repo_ensure_symlink already makes
its own parent directories and is idempotent, so nothing depended on
the guard; it is removed and the link is now attempted unconditionally.

Also stop swallowing a refused or failed link as success: the helper's
exit status is now checked, and agents-vault reports its own error and
exits 1 instead of silently continuing with no link in place.

Smaller fixes from the same review pass:
- check the exit status of _agents_repo_install_tools and the
  core.hooksPath git config write, instead of discarding both
- give the vmem mkdir failure a stderr message like every other fatal
  in the function
- guard hostname with type -q and add it to DEPENDENCIES
- .version creation now sets changed, so --link (which skips the
  commit step) reports it in --quiet mode
- reword --link's help/doc text: it still scaffolds the vault and
  links memory, it only skips the final commit
- drop the unused c_dim color variable
- move the __fish_agent_vault_dir / __fish_agent_vault_autopush
  documentation below Opinionated Components so its NOTE: callout
  (now flush-left so it actually renders as a Starlight Aside, per
  review) doesn't become the first Note aside in the page and steal
  the existing test's assertions about the original 4-bullet one

Adds two tests: pre-seeded vault entry with no live directory at all
(the restore path the guard was breaking), and a forced link failure
asserting agents-vault now exits 1 instead of 0.

docs/manual/07-customization.md

@@ -54,25 +54,6 @@ 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
@@ -178,6 +159,25 @@ interactively. See [Components Reference](/08-components-reference/) for the
 full sub-category breakdown of every category.
 
 
+## 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.
+
+
 ## Prompt and Theme
 
 ### Starship