feat(skills): add provider skill management (#909)

* feat(skills): add provider skill management

Users need one settings surface to discover and install skills without manually navigating provider-specific directories.

Add provider-backed global skill installation for Claude, Codex, Gemini, and Cursor, while keeping OpenCode read-only because it reuses other providers' skill locations.

Add a responsive Skills settings tab with scoped discovery, search, refresh controls, markdown and folder uploads, upload feedback, and overflow-safe layouts.

Validate bundled skill files and paths before writing them, preserve scripts and assets, and cover provider discovery and installation behavior with tests.

* fix(skills): preserve uploaded skill folders

Folder drops discarded supporting scripts and assets.

Keep relative paths and upload every file from the selected skill folder.

Use the selected folder name for installation and cover it in provider tests.

* fix(skills): restrict standalone skill uploads

Only show Markdown files when selecting standalone skills.

Normalize browser file paths so SKILL.md is not mistaken for a folder named dot.

* fix(skills): validate installs before writing

Preserve bundled files and normalize fallback names across skill installation paths.

Validate complete batches before writing and reject existing targets to avoid partial installs.

Keep project metadata and make folder selection tolerant of casing and cancelled dialogs.

* fix(skills): overwrite existing installations

Replace an existing skill directory instead of rejecting a duplicate installation.

Remove stale supporting files so the installed directory exactly matches the new upload.

server/shared/interfaces.ts

@@ -12,6 +12,7 @@ import type {
   ProviderModelsDefinition,
   ProviderMcpServer,
   ProviderSessionActiveModelChange,
+  ProviderSkillCreateInput,
   UpsertProviderMcpServerInput,
 } from '@/shared/types.js';
 
@@ -101,6 +102,15 @@ export interface IProviderSkills {
    * Lists all skills visible to this provider for the optional workspace.
    */
   listSkills(options?: ProviderSkillListOptions): Promise<ProviderSkill[]>;
+
+  /**
+   * Writes one or more global user-scoped skills for this provider.
+   *
+   * Implementations should install the supplied markdown entries into the
+   * provider's writable user skill folder and return the normalized skill
+   * records that were written.
+   */
+  addSkills(input: ProviderSkillCreateInput): Promise<ProviderSkill[]>;
 }
 
 // ---------------------------