Run the post-update hook and document hooks

docs/hooks.md

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+# Hooks
+
+Hooks are optional scripts you write to run your own commands when something in
+Blob happens. None ship active: `hooks/` holds only `.sample` templates, the
+installer copies them to `~/.config/blob/hooks/`, and `blob-hook` skips any file
+ending in `.sample`. Blob's own behaviour, such as recolouring apps on a theme
+change, is built into its scripts and never depends on a hook.
+
+To use one, copy the sample without the suffix and edit it:
+
+```
+cp ~/.config/blob/hooks/theme-set.sample ~/.config/blob/hooks/theme-set
+```
+
+`blob-hook <name>` runs `~/.config/blob/hooks/<name>` and then every file in
+`~/.config/blob/hooks/<name>.d/`, so several scripts can share one event. A
+failing hook prints a message and never stops the command that called it.
+
+| Hook | Run by | Argument |
+| --- | --- | --- |
+| `theme-set` | `blob-theme-set`, after a theme is applied | theme name |
+| `font-set` | `blob-font-set`, after the font changes | font name |
+| `battery-low` | `blob-battery-low` | battery percentage |
+| `post-update` | `blob-update`, after a successful update | none |