#74 fix: preserve resume target when spawning agent

Summary

• Preserve continueFrom when the dashboard server forwards /api/spawn requests through broker proxy mode.
• Return continueFrom and resumed from the mock spawn route so resume behavior is observable in tests.
• Update the resume-session regression test to assert the broker proxy route carries resume context.

Testing

• Added broker proxy coverage in packages/dashboard-server/src/proxy-server.test.ts for forwarding continueFrom to the broker /api/spawn endpoint.
• Updated test/bugs/bug7-resume-session.test.ts to assert resumed mock responses and static broker-proxy handling.

Notes

• Upstream origin is read-only for the authenticated GitHub account, so this PR is opened from the ship9599 fork.