install.sh: respect /ext disabled state on re-run

When linking, check for <name>.ts.off pointing into this repo and skip
relinking if found. Means a previously /ext-disabled extension stays
disabled across install.sh re-runs (e.g. when adding a new extension).

README + AGENTS updated with the new behavior.

AGENTS.md

@@ -176,6 +176,11 @@ effect without restarting pi.
 - **Symlink-friendly.** `fs.renameSync` renames the symlink itself; the repo
   target is untouched. Toggling an extension installed by this repo is
   reversible without re-running `install.sh`.
+- **`install.sh` respects the disabled state.** When linking, the installer
+  first checks for a `<name>.ts.off` symlink pointing into this repo and
+  skips re-linking if found. So re-running `./install.sh` (e.g. to pick up
+  a newly added extension) does not silently re-enable a previously
+  `/ext`-disabled extension.
 - **Subdir extensions are read-only in v1.** `name/index.ts` shapes show up in
   the listing with a `[dir]` tag but cannot be toggled — the cleanest disable
   for a directory would need a hidden-prefix or move-aside dance that adds