feat(tui): session resume — /resume command, exit hint, and dsh --resume <id>

Squashes feat/tui-resume-command, fix/tui-resume-desc, and
feat/tui-resume-flag.

packages/ui/app-boot/tests/app-boot.spec.ts

@@ -6,7 +6,7 @@ import { Context } from 'cordis'
 import SystemPrompt, { renderPrompt } from '@deepseek-ai/dsh-system-prompt'
 import {
   addHarnessSourceSection, assertEntriesLoaded, boot, HARNESS_SOURCE_SECTION,
-  installFailLoud, loadEnv, resolveConfigPath, type FailLoudProcess,
+  installFailLoud, loadEnv, parseResumeArg, resolveConfigPath, type FailLoudProcess,
 } from '../src/index.ts'
 
 const NAME = 'dsh-test-bin'
@@ -30,6 +30,26 @@ describe('resolveConfigPath', () => {
   })
 })
 
+describe('parseResumeArg', () => {
+  it('returns no resume id and passes arguments through when the flag is absent', () => {
+    expect(parseResumeArg([])).toEqual({ resumeSessionId: undefined, rest: [] })
+    expect(parseResumeArg(['custom.yml'])).toEqual({ resumeSessionId: undefined, rest: ['custom.yml'] })
+  })
+
+  it('parses the space form, the inline form, and leaves a positional config path in any position', () => {
+    expect(parseResumeArg(['--resume', 'sess-1'])).toEqual({ resumeSessionId: 'sess-1', rest: [] })
+    expect(parseResumeArg(['--resume=sess-2'])).toEqual({ resumeSessionId: 'sess-2', rest: [] })
+    expect(parseResumeArg(['--resume', 'sess-3', 'app.yml'])).toEqual({ resumeSessionId: 'sess-3', rest: ['app.yml'] })
+    expect(parseResumeArg(['app.yml', '--resume', 'sess-4'])).toEqual({ resumeSessionId: 'sess-4', rest: ['app.yml'] })
+  })
+
+  it('fails loud on a valueless, empty, or repeated flag rather than silently starting fresh', () => {
+    expect(() => parseResumeArg(['--resume'])).toThrow('--resume requires a session id')
+    expect(() => parseResumeArg(['--resume='])).toThrow('--resume requires a session id')
+    expect(() => parseResumeArg(['--resume', 'a', '--resume', 'b'])).toThrow('--resume may be given only once')
+  })
+})
+
 describe('loadEnv', () => {
   it('loads variables from .env in the given dir', () => {
     const dir = tmp()