Agentic UI: Add Skills tab to application settings

[bcotrim]

Related issues

• Related to STU-1882

How AI was used in this PR

Implemented with Claude Code from the STU-1882 series plan, porting the Skills panel from the exploration PR #3975/#3979 and reworking it per review feedback (toggles instead of Install/Remove buttons, optimistic updates). All code reviewed and visually verified in light + dark mode.

Proposed Changes

Adds a Skills tab to the Agentic UI application settings, letting users manage WordPress agent skills globally — applied to all existing and future sites.

• Each skill is a single row with a toggle: on installs it to every site, off removes it. State is optimistic, so toggles respond instantly and roll back if the operation fails.
• An Install all button in the panel header installs every remaining skill at once; it disappears once everything is installed (with reserved space, so the layout doesn't jump).
• Intro copy explains what skills are, with a Learn more link to the docs.

Light	Dark

Testing Instructions

1. Enable the Agentic UI and open Settings → Skills.
2. Toggle a skill on: within a moment, the skill appears under <site>/.agents/skills/<skill-id>/ for each site (e.g. ls ~/Studio/<site>/.agents/skills/) and its id is added to selectedSkills in ~/.studio/shared.json.
3. Toggle it off: the files and the selectedSkills entry are removed.
4. Click Install all: all remaining skills install sequentially; toggles disable and the button shows a loading state while it runs; the button disappears when everything is installed (no layout jump in the header, no flashing on individual toggles).
5. Verify light + dark mode.

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[wpmobilebot]

📊 Performance Test Results

Comparing cc285b3 vs trunk

app-size

Metric	trunk	cc285b3	Diff	Change
App Size (Mac)	1361.88 MB	1361.89 MB	+0.01 MB	⚪ 0.0%

site-editor

Metric	trunk	cc285b3	Diff	Change
load	1040 ms	1089 ms	+49 ms	⚪ 0.0%

site-startup

Metric	trunk	cc285b3	Diff	Change
siteCreation	6986 ms	7047 ms	+61 ms	🔴 0.9%
siteStartup	2373 ms	2387 ms	+14 ms	⚪ 0.0%

---

Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)

[nightnei]

Left 1 NIT comment.
Other than that, it looks and works peefectly

[shaunandrews]

I dig the toggles; nice work.