Expose runner workspace publication API over backend internals

[chubes4]

Problem

Homeboy Extensions should not call Data Machine or Data Machine Code publication/workspace abilities directly. WP Codebox owns the sandbox/runner boundary and should expose the runner workspace publication API that abstracts backend internals.

The immediate regression risk came from Extra-Chill/homeboy-extensions#1255, which correctly deleted fallback_pull_request plumbing but replaced it with a direct call to datamachine-code/publish-runner-workspace from Homeboy. That is a cleaner implementation than fallback PR plumbing, but still the wrong seam.

Desired boundary

Homeboy Extensions
  -> WP Codebox runner/publication API
    -> Data Machine / Data Machine Code / GitHub workspace backend internals

Not:

Homeboy Extensions
  -> Data Machine Code ability directly

Required API shape

Expose a WP Codebox-level API for publishing runner-owned workspace changes.

Inputs should include:

• runner workspace handle/path/backend identity
• target repo
• base/head branch context
• commit message
• PR title/body
• optional labels/draft/maintainer flags
• changed paths
• evidence/artifact context

Output should include typed metadata:

• success/failure
• failure type/error
• backend used
• branch/commit metadata
• PR URL/number and reused/opened state

Acceptance criteria

• Homeboy can publish a runner workspace without naming or calling Data Machine / Data Machine Code abilities directly.
• WP Codebox hides whether publication uses DMC, GitHub API workspaces, local git, or another backend.
• Missing publication support returns a typed failure that Homeboy can surface as write_without_pr.
• Smoke coverage proves the API forwards to the current DMC implementation where available.

Related

• DMC implementation currently exists in Add runner workspace publication API Extra-Chill/data-machine-code#626.
• Homeboy direct-call follow-up should remove datamachine-code/publish-runner-workspace from Extra-Chill/homeboy-extensions once this API exists.

[chubes4]

Architecture framing from Chris:

Data Machine is intended to be the invisible framework layer for complexity: robust custom database, composable architecture, and backend capabilities that can run inside ephemeral Playground contexts. WP Codebox combines those internals into clean, composable APIs. Homeboy and other orchestrators should talk to WP Codebox surfaces rather than reaching into Data Machine / Data Machine Code internals directly.

For this issue, the success criterion is not just wrapping the current DMC publication ability. The WP Codebox API should make the backend detail invisible to callers while still allowing the implementation to use DMC where that is the right substrate.