feat: add session resume capability with automatic continuation

Add ability to resume inactive sessions and automatically continue with new
session ID. Includes message merging for preserving conversation history,
UI improvements to allow sending during inactive state, and database schema
migration. close #87

cli/src/api/apiMachine.ts

@@ -101,7 +101,7 @@ export class ApiMachineClient {
 
     setRPCHandlers({ spawnSession, stopSession, requestShutdown }: MachineRpcHandlers): void {
         this.rpcHandlerManager.registerHandler('spawn-happy-session', async (params: any) => {
-            const { directory, sessionId, machineId, approvedNewDirectoryCreation, agent, model, yolo, token, sessionType, worktreeName } = params || {}
+            const { directory, sessionId, resumeSessionId, machineId, approvedNewDirectoryCreation, agent, model, yolo, token, sessionType, worktreeName } = params || {}
 
             if (!directory) {
                 throw new Error('Directory is required')
@@ -110,6 +110,7 @@ export class ApiMachineClient {
             const result = await spawnSession({
                 directory,
                 sessionId,
+                resumeSessionId,
                 machineId,
                 approvedNewDirectoryCreation,
                 agent,

cli/src/modules/common/rpcTypes.ts

@@ -2,6 +2,7 @@ export interface SpawnSessionOptions {
     machineId?: string
     directory: string
     sessionId?: string
+    resumeSessionId?: string
     approvedNewDirectoryCreation?: boolean
     agent?: 'claude' | 'codex' | 'gemini'
     model?: string

cli/src/runner/run.ts

@@ -328,11 +328,15 @@ export async function startRunner(): Promise<void> {
           : agent === 'gemini'
             ? 'gemini'
             : 'claude';
-        const args = [
-          agentCommand,
-          '--hapi-starting-mode', 'remote',
-          '--started-by', 'runner'
-        ];
+        const args = [agentCommand];
+        if (options.resumeSessionId) {
+            if (agent === 'codex') {
+                args.push('resume', options.resumeSessionId);
+            } else {
+                args.push('--resume', options.resumeSessionId);
+            }
+        }
+        args.push('--hapi-starting-mode', 'remote', '--started-by', 'runner');
         if (options.model) {
           args.push('--model', options.model);
         }
@@ -340,8 +344,7 @@ export async function startRunner(): Promise<void> {
           args.push('--yolo');
         }
 
-        // TODO: In future, sessionId could be used with --resume to continue existing sessions
-        // For now, we ignore it - each spawn creates a new session
+        // sessionId reserved for future use
         const MAX_TAIL_CHARS = 4000;
         let stderrTail = '';
         const appendTail = (current: string, chunk: Buffer | string): string => {