[agent builder] Add Manage skills sub-feature to permissions docs (#6861)

## Summary
- Adds `Manage skills` sub-feature to the Kibana privileges section with
inline `applies_to` for 9.4+
- Converts `Read` and `All` privilege descriptions from comma-separated
sentences to bulleted lists so each new 9.4+ item can carry its own
clear `applies_to` tag

Closes elastic/docs-content-internal#1257

## Reviewers
- @machadoum (elastic/kibana#256896 — Manage skills sub-feature)

## Test plan
- [ ] Verify `applies_to` annotations render correctly on individual
list items
- [ ] Confirm `Manage skills` privilege description matches the shipped
feature

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: leemthompo

explore-analyze/ai-features/agent-builder/permissions.md

@@ -37,16 +37,29 @@ This feature requires the appropriate {{stack}} [subscription](https://www.elast
 :::{applies-item} { stack: ga 9.4+, serverless: ga }
 #### `Read`
 
-Required to use agents, send chat messages, view tools, and access conversations.
+Grants access to:
+
+- Use agents
+- Send chat messages
+- View tools
+- View skills {applies_to}`stack: ga 9.4+`
+- Access conversations
 
 Instead of `All`, you can pair `Read` with individual sub-features for more granular control over what users can manage:
 
 - `Manage agents`: Create, update, or delete custom agents.
 - `Manage tools`: Create, update, or delete custom tools.
+- `Manage skills` {applies_to}`stack: ga 9.4+`: Create, update, or delete custom skills.
 
 #### `All`
 
-The broadest access level. Grants everything in `Read`, plus the ability to create, update, or delete custom agents and tools. Includes both management sub-features by default.
+The broadest access level. Grants everything in `Read`, plus the ability to:
+
+- Create, update, or delete custom agents
+- Create, update, or delete custom tools
+- Create, update, or delete custom skills {applies_to}`stack: ga 9.4+`
+
+Includes all management sub-features by default.
 :::
 
 :::{applies-item} { stack: ga 9.2-9.3 }