docs(maestro): switch INSTALL.md to the private-repo flow

`curl | sh` only works for public repos — gitea hides private repos with a
404 to unauthenticated clients. Lead with downloading `install.sh` via the
signed-in web UI (or `git show origin/main:install.sh`) and running it
locally. Note that the inner `cargo install --git` still uses whatever git
credentials the host already has, and document `MAESTRO_REPO_URL=ssh://...`
for SSH-only setups.

Refs NAP-225.

docs/INSTALL.md

@@ -1,19 +1,33 @@
 # Installing Maestro
 
-One command, used for both first install and subsequent updates:
+The repo is private, so grab `install.sh` while signed in to gitea, then run it:
 
-```sh
-curl -sSf https://git.naps.pt/naps62/maestro/raw/branch/main/install.sh | sh
-```
+1. Open <https://git.naps.pt/naps62/maestro/src/branch/main/install.sh>, click
+   **Raw**, save as `install.sh`. Or from any existing clone:
+   ```sh
+   git show origin/main:install.sh > install.sh
+   ```
+2. Run it:
+   ```sh
+   chmod +x install.sh && ./install.sh
+   ```
 
 The bootstrap script builds `maestro-installer` into a temp directory, runs the
 wizard, and cleans up — only `maestrod` and `maestro` end up persisted in
 `~/.cargo/bin`. Re-run any time to update; `cargo install` overwrites in place.
 
-Pass flags through to the wizard via `sh -s --`:
+Inside the script, `cargo install --git https://git.naps.pt/naps62/maestro …`
+uses your already-configured git credentials for this host (HTTPS helper or
+SSH agent). If only SSH is set up:
 
 ```sh
-curl -sSf https://git.naps.pt/naps62/maestro/raw/branch/main/install.sh | sh -s -- --yes --no-systemd
+MAESTRO_REPO_URL=ssh://git@git.naps.pt/naps62/maestro ./install.sh
+```
+
+Pass flags through to the wizard after `./install.sh`:
+
+```sh
+./install.sh --yes --no-systemd
 ```
 
 The wizard:
@@ -70,7 +84,7 @@ journalctl --user -u maestrod -f
 
 ## Running the wizard without the bootstrap script
 
-If you'd rather not pipe a script:
+If you'd rather skip the script:
 
 ```sh
 cargo install --git https://git.naps.pt/naps62/maestro maestro-installer