Add sibling command for installing and updating sibling projects

`sibling` asks for a git URL, clones the repo into
~/.local/share/siblings/<name>, runs its install.sh and confirms the
install. `sibling list` and `sibling remove` manage installed siblings.

updates.d/71-siblings.sh pulls every sibling during `update` and runs
its update.sh, reporting failures in the update summary.

Document the sibling contract in skills/sibling-project-format.md and
update the custom-hook guide to match.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

skills/sibling-project-updates.md

@@ -2,10 +2,41 @@
 
 ## Overview
 
-CoreDesktop's `update` script sources every `*.sh` file found in
-`~/.local/share/coredesktop/updates.d/` in filename order. A sibling project
-hooks into this system by installing a numbered shell script into that directory
-during its own install step.
+There are two ways for a sibling project to plug into CoreDesktop:
+
+1. **The `sibling` command (preferred).** The user runs `sibling` and gives it
+   the project's git URL. The repo is cloned into
+   `~/.local/share/siblings/<name>` and its `install.sh` (if any) is run once.
+   On every `update`, `updates.d/71-siblings.sh` pulls each sibling and runs its
+   `update.sh` (if any). The sibling only needs those two optional scripts.
+2. **A custom hook.** CoreDesktop's `update` script sources every `*.sh` file
+   found in `~/.local/share/coredesktop/updates.d/` in filename order. A sibling
+   project can install its own numbered shell script into that directory during
+   its own install step, for full control over how and when it updates.
+
+## Using the `sibling` Command
+
+See `sibling-project-format.md` for the full rules, templates and testing.
+In short, give the repo these optional files at its top level:
+
+| File         | When it runs                                      |
+|--------------|---------------------------------------------------|
+| `install.sh` | Once, from the repo folder, when the user runs `sibling add` (and again if they re-run it) |
+| `update.sh`  | On every `update`, from the repo folder, after the repo has been pulled |
+
+Both run with `bash`, and a non-zero exit is reported as a failure.
+`update.sh` should be idempotent. If you have no `update.sh`, `update` just pulls the repo.
+
+Commands:
+
+```bash
+sibling                 # asks for the git URL
+sibling add <url>       # same, without the prompt
+sibling list            # installed siblings, their URL and current commit
+sibling remove <name>   # deletes ~/.local/share/siblings/<name>
+```
+
+The rest of this document covers the custom-hook approach.
 
 ## How It Works
 
@@ -41,20 +72,21 @@ Existing coredesktop hooks use a numeric prefix to control execution order:
 | `60-pipx.sh`              | pipx packages                           |
 | `65-migrate.sh`           | Migrate backup tool                     |
 | `70-claude.sh`            | Claude Code setup                       |
+| `71-siblings.sh`          | Projects installed with `sibling`       |
 | `80-gnome-extensions.sh`  | GNOME extensions                        |
 | `85-gnome-settings.sh`    | GNOME extension settings                |
 | `90-firmware.sh`          | Firmware update check                   |
 | `95-cleanup.sh`           | Flatpak/pacman cache cleanup            |
 | `99-reboot-check.sh`      | Reboot-needed notice                    |
 
-Sibling projects should use an unused number from **71–79** so they run after
+Sibling projects should use an unused number from **72–79** so they run after
 the core updates but before coredesktop's end-of-run hooks (80+).
 
 ## What to Add to a Sibling Project
 
 ### 1. Create the hook script
 
-Add a file like `updates.d/70-myproject.sh` to the sibling repo:
+Add a file like `updates.d/72-myproject.sh` to the sibling repo:
 
 ```bash
 #!/bin/bash
@@ -73,12 +105,32 @@ In the sibling's `install.sh`, copy the hook into coredesktop's `updates.d/`:
 
 ```bash
 echo "Registering MyProject update hook..."
-cp updates.d/70-myproject.sh ~/.local/share/coredesktop/updates.d/
+cp updates.d/72-myproject.sh ~/.local/share/coredesktop/updates.d/
 ```
 
 This is a one-time step that wires the sibling into every future `update`
 run on the machine.
 
+### Optional: only update when installed
+
+Instead of running code at the top level, a hook can define `run_update` and,
+optionally, `is_installed`. `update` calls `run_update` only when `is_installed`
+is missing or returns true, so the hook is skipped on machines where the project
+isn't installed. Return non-zero from `run_update` to report a failure:
+
+```bash
+#!/bin/bash
+
+is_installed() {
+  [ -d "$HOME/.local/share/myproject" ]
+}
+
+run_update() {
+  echo "Updating MyProject..."
+  git -C "$HOME/.local/share/myproject" pull --ff-only
+}
+```
+
 ## Available Helpers
 
 Hooks don't inherit functions from earlier hooks, so source `utils.sh` for its
@@ -100,5 +152,5 @@ remove_packages "${MY_REMOVE_PACKAGES[@]}"
 - [ ] Create `updates.d/7X-<project>.sh` in the sibling repo
 - [ ] Hook script echoes its name so progress is visible during `update`
 - [ ] Install step copies the hook to `~/.local/share/coredesktop/updates.d/`
-- [ ] Numeric prefix is an unused number in 71–79
+- [ ] Numeric prefix is an unused number in 72–79
 - [ ] Script is idempotent — safe to run multiple times without side effects