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.

server/shared/types.ts

@@ -320,6 +320,47 @@ export type ProviderSkillListOptions = {
   workspacePath?: string;
 };
 
+/**
+ * One supporting file bundled with an uploaded provider skill.
+ *
+ * `relativePath` is resolved below the installed skill directory and must never
+ * be absolute or contain traversal segments. Text files may use `utf8`; binary
+ * scripts and assets should use `base64` so JSON transport does not corrupt
+ * their bytes.
+ */
+export type ProviderSkillCreateFile = {
+  relativePath: string;
+  content: string;
+  encoding: 'utf8' | 'base64';
+};
+
+/**
+ * One skill markdown payload submitted for provider-managed installation.
+ *
+ * `content` is the raw markdown body that will be written to `SKILL.md`.
+ * `directoryName` lets callers control the target folder name explicitly when
+ * they want stable filesystem paths that differ from the markdown front matter
+ * `name` field. `fileName` is optional upload metadata used only as a final
+ * fallback when no directory name or front matter name is present. `files`
+ * carries scripts, references, and other files from a complete skill folder.
+ */
+export type ProviderSkillCreateEntry = {
+  content: string;
+  directoryName?: string;
+  fileName?: string;
+  files?: ProviderSkillCreateFile[];
+};
+
+/**
+ * Shared input accepted by provider skill creation operations.
+ *
+ * The service layer batches multiple skill definitions in one request. Each
+ * entry can contain only markdown or a complete skill folder.
+ */
+export type ProviderSkillCreateInput = {
+  entries: ProviderSkillCreateEntry[];
+};
+
 /**
  * Normalized skill record returned by provider skill adapters.
  *