Implement Skills feature: filesystem-based skills with agent assignment

- Add SkillService: scans data/skills/ directory, parses _meta.json and SKILL.md frontmatter
- Add SkillEndpoint: REST API for listing skills and managing agent-skill assignments
- Add agent_skills DB table for agent-to-skill mapping (only relation in DB, skills live on filesystem)
- Extend SystemPromptBuilder: inject assigned skills' SKILL.md content into system prompt
- Extend ChatEndpoint + SessionEndpoint: load agent skills and pass to prompt builder
- Frontend: skill checkboxes in agent form, API functions for skill management
- Add form.css styling for skill list items

Services/DatabaseService.cs

@@ -134,6 +134,17 @@ public class DatabaseService
         TryAddColumn(conn, "agents", "use_identity", "INTEGER NOT NULL DEFAULT 1");
         TryAddColumn(conn, "agents", "use_user_info", "INTEGER NOT NULL DEFAULT 1");
         TryAddColumn(conn, "agents", "use_memory", "INTEGER NOT NULL DEFAULT 1");
+
+        conn.Execute(@"
+            CREATE TABLE IF NOT EXISTS agent_skills (
+                id            INTEGER PRIMARY KEY AUTOINCREMENT,
+                agent_id      TEXT NOT NULL,
+                skill_slug    TEXT NOT NULL,
+                created_at    TEXT NOT NULL,
+                UNIQUE(agent_id, skill_slug)
+            );
+            CREATE INDEX IF NOT EXISTS idx_agent_skills_agent ON agent_skills(agent_id);
+        ");
     }
 
     private void TryAddColumn(SqliteConnection conn, string table, string column, string type)
@@ -570,4 +581,51 @@ public class DatabaseService
             ORDER BY m.id",
             new { sessionId });
     }
+
+    // ── Agent Skills ──
+
+    public async Task<IEnumerable<string>> GetAgentSkills(string agentId)
+    {
+        using var conn = CreateConnection();
+        return await conn.QueryAsync<string>(
+            "SELECT skill_slug FROM agent_skills WHERE agent_id = @agentId ORDER BY skill_slug",
+            new { agentId });
+    }
+
+    public async Task SetAgentSkills(string agentId, IEnumerable<string> skillSlugs)
+    {
+        using var conn = CreateConnection();
+        using var tx = conn.BeginTransaction();
+
+        await conn.ExecuteAsync("DELETE FROM agent_skills WHERE agent_id = @agentId", new { agentId }, tx);
+
+        var now = DateTime.UtcNow.ToString("o");
+        foreach (var slug in skillSlugs)
+        {
+            await conn.ExecuteAsync(@"
+                INSERT INTO agent_skills (agent_id, skill_slug, created_at)
+                VALUES (@agentId, @slug, @now)",
+                new { agentId, slug, now }, tx);
+        }
+
+        tx.Commit();
+    }
+
+    public async Task AddAgentSkill(string agentId, string skillSlug)
+    {
+        using var conn = CreateConnection();
+        var now = DateTime.UtcNow.ToString("o");
+        await conn.ExecuteAsync(@"
+            INSERT OR IGNORE INTO agent_skills (agent_id, skill_slug, created_at)
+            VALUES (@agentId, @skillSlug, @now)",
+            new { agentId, skillSlug, now });
+    }
+
+    public async Task RemoveAgentSkill(string agentId, string skillSlug)
+    {
+        using var conn = CreateConnection();
+        await conn.ExecuteAsync(
+            "DELETE FROM agent_skills WHERE agent_id = @agentId AND skill_slug = @skillSlug",
+            new { agentId, skillSlug });
+    }
 }