feat: add slash command autocomplete to HappyComposer

Implements full-stack slash command autocomplete with agent-specific built-in commands and user-defined command discovery. Includes React Strict Mode fix for suggestion handling.

server/src/sync/syncEngine.ts

@@ -133,6 +133,18 @@ export type RpcReadFileResponse = {
     error?: string
 }
 
+export type SlashCommand = {
+    name: string
+    description?: string
+    source: 'builtin' | 'user'
+}
+
+export type RpcSlashCommandsResponse = {
+    success: boolean
+    commands?: SlashCommand[]
+    error?: string
+}
+
 export type SyncEventType =
     | 'session-added'
     | 'session-updated'
@@ -705,6 +717,10 @@ export class SyncEngine {
         return await this.sessionRpc(sessionId, 'ripgrep', { args, cwd }) as RpcCommandResponse
     }
 
+    async listSlashCommands(sessionId: string, agent: string): Promise<RpcSlashCommandsResponse> {
+        return await this.sessionRpc(sessionId, 'listSlashCommands', { agent }) as RpcSlashCommandsResponse
+    }
+
     private async sessionRpc(sessionId: string, method: string, params: unknown): Promise<unknown> {
         return await this.rpcCall(`${sessionId}:${method}`, params)
     }

server/src/web/routes/sessions.ts

@@ -182,5 +182,31 @@ export function createSessionsRoutes(getSyncEngine: () => SyncEngine | null): Ho
         return c.json({ ok: true })
     })
 
+    app.get('/sessions/:id/slash-commands', async (c) => {
+        const engine = requireSyncEngine(c, getSyncEngine)
+        if (engine instanceof Response) {
+            return engine
+        }
+
+        // Session must exist but doesn't need to be active
+        const sessionResult = requireSessionFromParam(c, engine)
+        if (sessionResult instanceof Response) {
+            return sessionResult
+        }
+
+        // Get agent type from session metadata, default to 'claude'
+        const agent = sessionResult.session.metadata?.flavor ?? 'claude'
+
+        try {
+            const result = await engine.listSlashCommands(sessionResult.sessionId, agent)
+            return c.json(result)
+        } catch (error) {
+            return c.json({
+                success: false,
+                error: error instanceof Error ? error.message : 'Failed to list slash commands'
+            })
+        }
+    })
+
     return app
 }