feat: sync open-source context and capsule removal

- remove legacy contexts surfaces from the app, MCP bridge, standalone server, and docs
- keep getContext and retrieveQueryContext while aligning the simplified graph-only contract
- harden dev startup migration behavior and disable the accidental chat surface in open source

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src/tools/skills/writeSkill.ts

@@ -0,0 +1,38 @@
+import { tool } from 'ai';
+import { z } from 'zod';
+import { writeSkill } from '@/services/skills/skillService';
+import { eventBroadcaster } from '@/services/events';
+
+export const writeSkillTool = tool({
+  description: 'Write or update a skill. Content should be full markdown with YAML frontmatter (name, description).',
+  inputSchema: z.object({
+    name: z.string().describe('The name of the skill to write'),
+    content: z.string().describe('Full markdown content including YAML frontmatter'),
+  }),
+  execute: async ({ name, content }) => {
+    try {
+      const result = writeSkill(name, content);
+      if (!result.success) {
+        return {
+          success: false,
+          error: result.error,
+          data: null,
+        };
+      }
+
+      eventBroadcaster.broadcast({ type: 'GUIDE_UPDATED', data: { name } });
+
+      return {
+        success: true,
+        data: { name },
+        message: `Skill "${name}" saved`,
+      };
+    } catch (error) {
+      return {
+        success: false,
+        error: error instanceof Error ? error.message : 'Failed to write skill',
+        data: null,
+      };
+    }
+  },
+});