feat(build): add BuildManager extension and trim BuildStatus enum

[behinddwalls]

Summary

Adds a vendor-agnostic BuildManager interface under extension/build
so the orchestrator can drive builds against an external provider
(BuildKite, Jenkins, an internal job runner, etc.) without coupling to
any one of them.

Interface

BuildManager exposes four verbs — Trigger, Status, Cancel,
Close:

• Trigger returns (buildID, status, error); an implementation that
  can deduce an already-finished build returns its terminal status so
  the caller can skip the first Status poll, otherwise Accepted.
• Status may be synchronous and lengthy (a provider round trip);
  Trigger/Cancel must return promptly.
• The godoc spells out what is required of an implementation:
  concurrency safety, internal recovery from transient connectivity
  failures, transient-only local state, and plain errors left for the
  controller to classify per core/errs. No custom error sentinels
  ship yet — they land with the first implementation that needs them.

Entity changes

• entity.BuildStatus is
  {Unknown, Accepted, Running, Succeeded, Failed, Cancelled}. This
  drops the provider-specific Queued/Blocked stages and renames
  Passed → Succeeded, while keeping Running as a first-class
  non-terminal state so callers can distinguish a queued/accepted build
  from one that is actively executing. Orchestrator build/buildsignal
  references use Accepted.
• Adds entity.ChangeAction, entity.BuildChange,
  entity.BuildMetadata as Trigger inputs/outputs.

Out of scope

No backend implementation and no orchestrator wiring — those are
follow-ups.

Test Plan

• ✅ `make fmt && make lint`
• ✅ `make check-tidy && make check-gazelle && make check-mocks`
• ✅ `bazel build //...`
• ✅ `bazel test //... --test_tag_filters=-integration`
  (integration tests require Docker, not run here)

Issues

Stack

1. @ feat(build): add BuildManager extension and trim BuildStatus enum #167
2. feat(build): add noop BuildManager implementation #168
3. feat(orchestrator): wire BuildManager into the build controller #169