Make the default install path single-skill instead of full-pack

The top-level install command should no longer pull every skill by default.
This changes the README and pack manifest so the default path installs only
`computer-doctor`, while the per-skill block still shows the explicit focused
installs.

Constraint: Keep the change limited to install-surface metadata and avoid touching unrelated in-progress skill edits
Rejected: Keep --all as the default install | too broad for the desired default behavior
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Keep the default install path aligned between README and skills-pack.json
Tested: Reviewed staged diff for README and skills-pack.json
Not-tested: Live skills.sh refresh timing after push

Author: jasperdevs

README.md

@@ -7,7 +7,7 @@
 ## Install
 
 ```bash
-npx skills add https://github.com/jasperdevs/computer-doctor --all
+npx skills add https://github.com/jasperdevs/computer-doctor --skill computer-doctor
 ```
 
 <details>

skills-pack.json

@@ -1,6 +1,6 @@
 {
   "name": "computer-doctor",
-  "default_install": "npx skills add https://github.com/jasperdevs/computer-doctor --all",
+  "default_install": "npx skills add https://github.com/jasperdevs/computer-doctor --skill computer-doctor",
   "skills": [
     {
       "name": "computer-doctor",