Resolve Docs project-backed opener sources

[ymichael]

What was wrong

PR #2088 correctly preserved PluginFileOpenerSource.experimental_hostId through the Docs frontend and private RPC, but resolveOpenerFile consumed it only for source.kind === "host". Root Compose opens a project-source Markdown file as { kind: "workspace", environmentId: null, projectId, experimental_hostId }, so Docs accepted the source identity and then skipped its only workspace branch, throwing “Docs can open workspace, host, and thread-storage files only.” The same project-backed file already works in the PDF opener through the core project content route.

What changed

Docs now resolves a workspace source with no environment and a project ID through the project's authoritative local-path source. An explicit experimental_hostId selects that host; omission resolves through system.config().primaryHostId, matching the public opener contract and core project-workspace routing. The resolved source root and host feed the existing confined Files SDK read, preview, and optimistic-write calls. Missing, duplicate, or non-absolute project-source identities fail closed instead of selecting another machine.

This is the smallest complete fix: one resolver branch plus one server regression. Environment-backed workspace, absolute host-file, and thread-storage behavior is unchanged. There is no SDK, CLI, guide, configuration, or host-daemon wire change, so HOST_DAEMON_PROTOCOL_VERSION is unchanged.

How you verified

The pre-fix focused regression failed both project-backed cases with the reported error (2 failed, 31 passed):

• pnpm exec turbo run test --filter=bb-plugin-simple-notes -- server.test.ts

After the fix and the final rebase onto deb723091 / merged #2059:

• The focused server suite passed 32 tests. The regression proves selected-host read, preview, and optimistic save routing; omitted-host primary routing even when a different project source is marked default; and rejection of an unknown selected host before file I/O.
• pnpm exec turbo run test typecheck build --filter=bb-plugin-simple-notes --force passed: 66 tests, Docs build and typecheck, 7 successful Turbo tasks.
• git diff --check origin/main...HEAD passed.

Related to #2088, #2083, and #2059.

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