[alexandre-vl]

Human comments

What was wrong

A thread created with no environment (project-default) and no live parent always resolved to the primary host. Examples are a Tasks preset, an automation run, or bb thread spawn without an environment flag. resolveProjectDefaultThreadEnvironment took requireConnectedPrimaryHostId and then looked up the project's checkout on that host. It never checked that the project had a checkout there. A project whose only checkout lives on another machine, such as a laptop, therefore failed with 404 Project has no local-path source for host (#4446).

What changed

• thread-default-policy.ts: the new resolveProjectDefaultHostId keeps the primary host whenever it has a checkout of the project, so today's placement is unchanged for those projects. Otherwise it takes a persistent machine that holds a checkout. It skips destroyed and removing machines, and prefers the project's default source, then the oldest one. That machine is readied the way an explicitly chosen one is (ensureHostSessionReadyForWork):
  • a suspended machine is woken;
  • an offline machine answers 502, so the thread never silently moves to another machine.
• project-sources.ts: getProjectSourceOnPersistentHost, one join query.
• bb guide threads says where a thread with no environment flag starts.
• No server/daemon wire change, so no HOST_DAEMON_PROTOCOL_VERSION bump.

How you verified

• A new project-default machine block runs against the real server and a migrated SQLite database. Each test creates a project-default thread through the public API. The "main" column comes from running the same file against main's server code (b143a2286): 4 tests failed and 2 passed.

  Project checkouts	main	This PR
  Only on a laptop (not the primary)	404, expected 404 to be 201	201, worktree on the laptop
  On the primary and a laptop, default source on the laptop	201 on the primary	201 on the primary
  Default source on a temporary machine, another on a laptop	404, expected 404 to be 201	201 on the laptop
  On an offline laptop	404, expected 404 to be 502	502, no fallback to the primary
  On a suspended laptop	404, expected 404 to be 201	201, laptop woken
  Only on a temporary machine	404	404

  If the fix only requires a connected machine instead of waking it, the suspended-machine test fails with expected 502 to be 201.

• pnpm exec turbo run typecheck lint --filter=@bb/server passed.

• The six server suites that cover project-default placement passed (177 tests): thread-default-policy, environment-providers, public-threads.project-default, thread-runtime-config, provider-management and plugin-sdk.

• Before the wake was added, pnpm exec turbo run typecheck test --filter=@bb/server --filter=@bb/db --filter=@bb/templates passed @bb/server 3631, @bb/db 632 and @bb/templates 46.

• No UI change, so there are no screenshots; the status codes above are the before and after. I did not reproduce it in a live instance, which needs a second enrolled machine.

Fixes #4446

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