broaden install.sh first-run guidance about reinstall safety

[cjus]

Summary

The existing first-run text after install said only SOUL.md/SOLRAC.md edits survive reinstalls, which undersells what's actually safe. install.sh only ever writes one file (~/.solrac/bin/solrac); every other file under ~/.solrac/ is untouched.

Before

3. run `solrac`. SOUL.md, SOLRAC.md, and data/ will appear in /Users/cjus/.solrac
   on first boot. edits to SOUL.md/SOLRAC.md survive future reinstalls of
   this script — only the binary at /Users/cjus/.solrac/bin/solrac is replaced on update.

After

3. run `solrac`. SOUL.md, SOLRAC.md, and data/ will appear in /Users/cjus/.solrac
   on first boot. reinstalling this script ONLY replaces the binary at
   /Users/cjus/.solrac/bin/solrac — everything else under /Users/cjus/.solrac
   (SOUL.md, SOLRAC.md, .env, data/, skills/, tasks/, integrations/, and
   any other files you add) is left untouched.

Why

Surfaced when verifying first-run UX of v0.3.0-rc1. An operator with a sizable skills//tasks/ layout would reasonably wonder if those survive — they do, and the install text should say so.

Behavior

No code change. Documentation only.

Sync convention

After merge, copy the updated scripts/install.sh to cjus-dev/public/solrac/install.sh (per the sync convention from PR cjus-dev #1).

Test plan

• After merge, copy script to cjus-dev, redeploy, and verify curl -fsSL https://cjus.dev/solrac/install.sh | sh prints the new line on a clean install.