feat(ssh): wave 3 — session-orchestrator computerId threading + transport-contract API types

Wave 3 of transparent SSH support (2 parallel owner-agents on disjoint packages).

- session-orchestrator: thread computerId end-to-end through the turn, mirroring
  cwd exactly — StartTurnInput/EnqueueInput/handleMessage/TurnLifecyclePayload
  gain computerId; runTurnDetached resolves effectiveComputerId via
  conversationStore.getEffectiveComputer(convId, override), persists the override,
  threads into RunTurnInput + ToolAssembly. Register a remote-degradation
  tools-filter (filterRemoteIncompatibleTools) that, when assembly.computerId is
  set (REMOTE), drops the 'lsp' tool + any '__'-namespaced MCP tool (local
  processes that can't see remote files); LOCAL (computerId undefined) is a
  passthrough — byte-identical to today. +21 tests.
- transport-contract: + computerId on ChatRequest (flows to ChatSendMessage) +
  computer endpoint API types (ComputerListResponse, ComputerResponse,
  ComputerStatusResponse, SetConversationComputerRequest,
  ConversationComputerResponse, SetWorkspaceDefaultComputerRequest,
  TestComputerResponse) — mirrors the cwd/workspace endpoint types.
- CR-1 (non-blocking, folded into wave 4): MCP filter doesn't preserve computerId
  on the returned ToolAssembly.
- cache-warming computerId threading intentionally DEFERRED (user request) —
  noted as a known performance-only limitation in tasks.md.

Verified: tsc -b EXIT 0, biome clean, 1620 vitest pass (was 1599, +21).
Refs: notes/ssh-support-plan.md (decisions §0.5/§13). No merge or push.

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