Capture BuildKit output in build logs

[phinze]

Fixes MIR-658

Build logs were only showing the detection phase stuff, but all the
juicy BuildKit output (layer pulls, cache status, command output) that
you see during miren deploy was missing from miren logs --build.

Turns out we had two WithStatusUpdates callbacks - one for persisting
logs, one for streaming to the client - but WithStatusUpdates just
overwrites, it doesn't chain. So the streaming callback was clobbering
the persistence one.

Combined them into a single callback that does both. Now build logs
include everything: layer info, phase markers, cache status, and
importantly the actual command output (like bun install or go build
output, and onbuild commands).

[coderabbitai]

📝 Walkthrough

Walkthrough

The change refactors servers/build/build.go to deduplicate and scope BuildKit vertex log entries by tracking vertexStarted and vertexCompleted events. It replaces the previous per-status callback with a single combined status callback that: logs vertex start once, logs vertex completion when cached once, writes per-line BuildKit output (trimming whitespace) to the build log, and forwards the raw SolveStatus payload to the status client when connected. Vertex tracking is initialized before status callbacks are registered; existing phase-based status messages are preserved.

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[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@servers/build/build.go`:
- Around line 770-779: The code trims leading whitespace when logging build
output (in the loop over ss.Logs where lines are processed and passed to
buildLog.write), which destroys indentation for stack traces and formatted
output; replace the call to strings.TrimSpace(line) with a trailing-only trim
(use strings.TrimRight(line, "\r") or equivalent) so you only strip Windows CR
characters but preserve leading spaces before calling buildLog.write.

[evanphx]

aaaaaaah!