feat(server): allow concurrent hosts

[Rinat S (rsolmano)]

Summary

Allow multiple ThinkRail hosts to start against the same canonical data directory. This removes the startup collision that prevented running bun run dev from the main checkout and feature workspaces at the same time while preserving each launcher's existing port selection and graceful shutdown.

Changes

• remove the data-directory ownership listener and its startup error types
• simplify bootHost to retain process signal handling without lease lifecycle wrapping
• replace second-host refusal coverage with a regression test that boots and health-checks two hosts against one data directory
• remove the obsolete target-native ownership check from the release workflow
• update architecture, server, CLI, desktop, and CI specs to record the new policy

Concurrent hosts still have independent in-memory services and event streams. Shared persistence has no cross-process locking or convergence guarantee, so simultaneous writes may be last-writer-wins.

Testing

• bun run check:deps — passed
• bun run check:boundaries — passed
• bun run check:seams — passed
• bun run lint — passed with 6 pre-existing unused-suppression warnings
• bun run typecheck — 14/14 tasks passed
• bun run test — 14/14 package tasks passed; server reported 871 tests passed
• bun run e2e — 316 tests passed across 8 shards
• two-process shared-data smoke — both hosts started on distinct ports and returned ok from /health

[jetbrains-air]

Approved — ready to merge.