feat(paths): add resolveSessionsRoot as the one shared session-store root

Every surface that persists or lists sessions resolves one directory under
the Harness home, so history is shared across working directories instead
of scattered per project.

packages/util/paths/tests/paths.spec.ts

@@ -4,10 +4,12 @@ import { describe, expect, it } from 'vitest'
 import {
   DEFAULT_DSH_HOME_DISPLAY,
   DSH_HOME_DIR_NAME,
+  SESSIONS_DIR_NAME,
   defaultDshHome,
   dshHomeDisplay,
   expandHomePath,
   resolveDshHome,
+  resolveSessionsRoot,
 } from '@deepseek-ai/dsh-paths'
 
 describe('dsh path helpers', () => {
@@ -38,6 +40,15 @@ describe('dsh path helpers', () => {
     expect(resolveDshHome(undefined, { DSH_HOME: '   ' })).toBe(defaultDshHome())
   })
 
+  it('resolves the session store under the home it was given, by the same precedence', () => {
+    expect(SESSIONS_DIR_NAME).toBe('sessions')
+    expect(resolveSessionsRoot('/tmp/explicit-dsh', { DSH_HOME: '~/env-dsh' }))
+      .toBe(join(resolve('/tmp/explicit-dsh'), 'sessions'))
+    expect(resolveSessionsRoot(undefined, { DSH_HOME: '~/env-dsh' }))
+      .toBe(join(homedir(), 'env-dsh', 'sessions'))
+    expect(resolveSessionsRoot(undefined, {})).toBe(join(defaultDshHome(), 'sessions'))
+  })
+
   it('labels a resolved home by whether it is the default root', () => {
     expect(dshHomeDisplay(resolve(defaultDshHome()))).toBe('~/.dsh')
     expect(dshHomeDisplay('/some/other/root')).toBe('$DSH_HOME')