feat(core): add remote workspace lifecycle seam

[kitlangton]

What

Introduce the provider-neutral lifecycle and environment seam for centrally executed V2 Sessions to address hosted Workspaces without touching the server host checkout.

• Adds browser-safe Workspace.Info metadata.
• Adds provider-neutral create, connect, suspend, reconcile, and delete contracts.
• Adds a lazy, scoped, idle-expiring Workspace connection cache.
• Fences lifecycle transitions against active file and process leases.
• Resolves hosted Location project metadata from the Workspace record instead of host-local Project.resolve.
• Keeps implicit-local Location behavior unchanged.
• Includes the staged implementation plan in specs/v2/remote-workspace-execution.md.

How

• packages/core/src/workspace.ts owns Workspace creation, connection, suspension, and removal; compensates failed initial persistence; persists replacement bindings before reconciliation; and keeps deletion retryable when the provider fails.
• A keyed writer-preferring lifecycle gate closes admission, drains scoped borrows, runs one transition, and reopens admission on success, failure, or interruption.
• packages/core/src/workspace.ts reads durable metadata independently from provider connection, shares concurrent connections through RcMap, and immediately evicts failed acquisitions.
• packages/core/src/workspace/sandbox.ts requires a versioned kind: "sandbox" placement envelope before V2 can interpret legacy Workspace JSON. Providers own binding decoding and lifecycle operations.
• packages/core/src/workspace/environment.ts binds separate local and hosted Location services. Hosted file operations borrow one connection, resolve provider-canonical paths, and reject direct or symlink escapes outside the Workspace root.
• packages/core/src/location.ts selects local Project resolution or hosted Workspace metadata at node-binding time and lexically rejects working directories outside the durable Workspace root.
• Local mutations share one keyed target lock, including ordinary writes, conditional writes, exclusive creates, and removal.

Scope

This PR establishes placement, lifecycle, and connection seams only.

• No Vercel, Daytona, Modal, or E2B production adapter.
• No public HTTP or product provisioning surface for hosted Workspaces.
• No migration of existing file, shell, search, config, VCS, PTY, snapshot, or watcher consumers.
• No claim that a complete hosted Session can execute tools yet.
• Hosted Location lookup errors still follow the current infallible Location-layer boot contract; transport-level placement errors are deferred with the public Workspace API surface.

Testing

• bun run test test/workspace.test.ts test/location.test.ts test/location-layer.test.ts: 20 passed.
• bun run test --rerun-each 10 test/workspace.test.ts: 10 consecutive lifecycle runs passed.
• Lifecycle coverage includes stable create identity, failed-persistence compensation, shared scoped connections, suspend draining and admission fencing, replacement-binding reconnection, retryable provider deletion, and final row removal.
• Live Modal SDK 1.5.2 tracer verified Sandbox creation, reconnection by ID, direct file operations, foreground commands, timeout cleanup, filesystem snapshot and restoration, retained writable files, credential isolation, and Sandbox cleanup.
• Modal timeout verification observed the command gone while the Python SDK resolved with -1; current Python and JavaScript process handles expose no direct per-process termination.
• bun typecheck in packages/core, packages/schema, packages/protocol, packages/server, packages/sdk-next, and packages/opencode.
• Push hook: bun turbo typecheck --concurrency=3, 32 configured tasks passed.
• bun run migration --check in packages/core.
• Prettier and git diff --check passed.
• Simplify review completed with separate reuse, quality, and efficiency passes, followed by a focused Effect lifecycle review.
• Full Core run: 1,323 passed, 6 skipped, 10 failed. The failures are 9 existing provider-opencode five-second timeouts and 1 provider-openai polling timeout; both reproduce independently and do not touch Workspace or Location code.

Flow

sequenceDiagram
  participant L as Location graph
  participant W as Workspace lifecycle
  participant G as Lifecycle gate
  participant C as Connection cache
  participant P as Sandbox provider

  L->>W: get(workspaceID)
  W-->>L: project and root metadata
  Note over L,P: No provider connection during Location boot
  L->>G: borrow read lease
  G->>C: acquire shared connection
  C->>P: decode and connect on cache miss
  P-->>C: environment and replacement binding
  C->>P: reconcile(binding)
  C-->>L: hosted environment
  W->>G: suspend or remove
  G-->>W: wait for active leases, block new leases
  W->>P: transition provider state
  W->>W: persist replacement binding
  W->>G: reopen admission

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