refactor: Decompose ipc_server.rs into module directory (#139)

Extract streaming protocol handlers (attach, grid, status) into
ipc_server/streams.rs and tests into ipc_server/tests.rs, isolating
the highest-churn code from server lifecycle and dispatch logic.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: 2witstudios