feat(docs): split fish-config.md into docs/manual SSOT tree

docs/manual/09-installation.md

@@ -0,0 +1,43 @@
+---
+title: Installation
+manTitle: 9. INSTALLATION
+sidebar:
+  order: 9
+helpKeywords:
+- installation
+- install
+---
+
+This configuration is managed as a git repository. To deploy on a new machine:
+
+    mv ~/.config/fish ~/.config/fish.bak   # back up any existing config
+    git clone https://git.rootiest.dev/rootiest/fish-config.git ~/.config/fish
+
+Then open a new Fish shell. Fisher installs automatically on first launch
+and the Catppuccin Mocha theme is applied. All other plugin functionality is
+bundled directly with this config and requires no additional installation.
+
+## Return Sentinel
+
+config.fish ends with a return sentinel guard. Any lines appended after it by
+a tool's setup command (starship init fish | source, zoxide init fish | source,
+etc.) will have no effect. All integrations are managed via conf.d/ files.
+
+If a new tool's shell integration appears to do nothing, check whether its
+setup command appended an init line below the sentinel and create a dedicated
+conf.d/<tool>.fish instead.
+
+## Updating
+
+Pull the latest changes from the upstream repository without needing a
+configured git remote:
+
+    config-update              Fetch and apply the latest commits from upstream
+    config-update --dry-run    Preview available changes without applying them
+    config-update --force      Stash local changes, pull, then restore the stash
+
+The remote URL (https://git.rootiest.dev/rootiest/fish-config.git) is
+hard-coded, so this works on a fresh clone with no origin configured. All
+git output is suppressed. Run exec fish after a successful update to reload.
+
+---