feat(skills): import SKILL.md bundles from public GitHub URLs (#2576)

* feat(skills): import SKILL.md bundles from public GitHub URLs

Supports GitHub tree/blob/raw links and skills.sh pages that resolve to GitHub.
Installs SKILL.md plus sibling text assets under data/skills/imported/.

Co-authored-by: Cursor <cursoragent@cursor.com>

* fix(skills): admin-gate URL import and validate redirect hosts

- require_admin on POST /api/skills/import-from-url (matches other skill admin routes)
- reject cross-host redirects after httpx follow_redirects
- test for redirect host validation

Co-authored-by: Cursor <cursoragent@cursor.com>

* fix(skills): match Brain Add panel import/submit button styles

- Skill URL Import: theme-io-btn + download icon (same as memory Import)
- Add Skill submit: confirm-btn confirm-btn-primary

Co-authored-by: Cursor <cursoragent@cursor.com>

* fix(skills): allow api.github.com during directory import

Real imports hit the GitHub contents API after redirects; whitelist
api.github.com and add regression tests. Shrink Import button with flex:none.

Co-authored-by: Cursor <cursoragent@cursor.com>

* fix(skills): align skill Import button with URL input row

Match memory-add-input height (28px) in memory-add-row and center the
download icon with flexbox instead of vertical-align hacks.

Co-authored-by: Cursor <cursoragent@cursor.com>

* fix(skills): cancel modal-body margin on skill Import button

The skill Import button sits in .memory-add-row beside an input; the
global .modal-body button { margin-top: 6px } rule only affected buttons,
pushing Import down and misaligning the download icon. Reset margin-top
and match Memory Import SVG markup at 28px row height.

Co-authored-by: Cursor <cursoragent@cursor.com>

* fix(skills): surface GitHub API errors on URL import

Pass through GitHub response messages (especially 403 rate limits) as
SkillImportError instead of a generic download failure.

Co-authored-by: Cursor <cursoragent@cursor.com>

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

static/style.css

@@ -10126,6 +10126,15 @@ details a:hover {
   height: 32px;
 }
 
+/* Skill Import beside URL field — match input height; cancel modal-body button margin. */
+.memory-add-row .theme-io-btn {
+  flex: none;
+  height: 28px;
+  box-sizing: border-box;
+  margin-top: 0;
+  padding: 5px 10px;
+}
+
 .memory-add-input {
   flex: 1;
   height: 28px;