Expose attach subpaths and sandbox readonly requests (spawn contract pending)

[agent-relay-code]

Summary

Draft implementation of the attach and read-only-request portions of the workforce#338 upstream work. This does not yet unblock workforce#338. The reviewed plan's D4 decision is pending: spawnFleetSandbox must either orchestrate a running agent or expose a narrower ensure-only contract. No placeholder spawnFleetSandbox is exported.

• Move the attach adapter and workspace transport resolution into Cloud, retaining CLI re-exports and existing broker transport fields. Add Cloud and SDK /attach entries, nodeId/optional-agent discovery, a private single-client raw stdio socket, idempotent cleanup, and a completion promise.
• Expose existing provisioning primitives through Cloud and SDK /fleet; route the CLI's ensure import through the SDK entry. Preserve existing Cloud enrollment APIs. The running-agent spawn wrapper remains pending D4.
• Validate and forward readonlyPaths as explicit /path/** subtrees; expose fleet spawn --sandbox-readonly-path with mount guards.
• Add socket, request-validation, CLI, and built-entry regression coverage. Add an independent read-only-request RelayFlow proof; leave historical case 1630 unchanged.

The proxy retains broker transport fields alongside socketPath/finished/close, as the reviewed plan recommends: workforce duck-types the three consumer fields and ignores extras. Completion is an inferred status (0 for normal terminal closure/detach, 1 for transport failure), not a remote harness exit code. SDK re-exports add a dependency on Cloud and its transitive dependencies. No package versions changed.

Remaining requirements

• Resolve reviewed-plan.md D4 and implement/test spawnFleetSandbox registration, placement confirmation, input mapping, and teardown.
• Cloud's server-side ensure handler/mount builder is absent from this repository. Forwarding does not enforce chmod-444; server implementation and a live Daytona write-denial proof are still required.
• Exports follow the reviewed plan's existing ESM conventions. No CommonJS build or require condition was added.
• No live CLI sandbox-spawn or node-attach parity run was performed.

Validation

• npm run typecheck: passed.
• npm run build: passed (Rust build skipped automatically because Cargo is unavailable).
• SDK suite: 182 passed; fleet suite: 43 passed.
• Cloud suite: 450 passed, 4 skipped after removing inherited Cloud authentication variables.
• Existing and new attach tests: 42 passed, using actual local WebSocket and UNIX socket servers.
• CLI suite via npx vitest run packages/cli/ --maxWorkers=2: 1,652 passed, 29 skipped. npm --prefix packages/cli test is unavailable because that package has no test script. Tests ran without inherited Cloud/Relay credentials and with local Git file transport enabled for the temporary-repository fixture.
• Built Cloud/SDK entry-resolution regression: passed.
• New proof runner executed locally against base c978302 and implementation b6c3bb1: base absent, head fixed. Both generated probes and compiled CLI-help checks passed. This was not a hosted Cloud or live Daytona run.

RelayFlow Proof

• Change type: feature
• RelayFlow case: sandbox-readonly-paths

This generated base/head probe verifies exact request forwarding and compiled CLI help. It does not provision a sandbox or prove chmod enforcement.

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