internal/process: surface upstream output on premature exit

[davidbarnhart]

When a model's process exits before becoming ready, clients received the
opaque "upstream command exited prematurely" with none of the upstream
server's output, so a bad launch (for example an invalid argument) could not
be diagnosed from the client. Include the captured process log so the real
cause reaches the caller; because WaitReady subscribers share the start
result, every waiting request benefits too.

• prematureExit now appends processLogger.GetHistory() when non-empty
• clear the process log before each launch so only the current attempt's
  output is surfaced, not accumulated history from earlier launches

fixes #896

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[greptile-apps]

Greptile Summary

This PR surfaces the captured process output in the "upstream command exited prematurely" error, making startup failures diagnosable from the client. It also clears the circular log buffer before each launch so only the current attempt's output appears in the error.

• prematureExit now appends processLogger.GetHistory() to the error message when the buffer is non-empty, and all WaitReady subscribers share this richer result.
• processLogger.Clear() is called immediately before cmd.Start() so accumulated output from earlier restarts cannot bleed into a new attempt's failure message.

Confidence Score: 5/5

Safe to merge. The change is small and well-scoped: two call sites in doStart, both guarded by the existing cmdDone-closed invariant that ensures all subprocess I/O has drained before GetHistory is read.

Clear() is placed before cmd.Start() so no subprocess output can arrive before the buffer is reset. GetHistory() is called only inside prematureExit(), which is reachable only after cmdDone closes — and cmdDone is closed after cmd.Wait() returns, which drains all I/O goroutines. The bufferMu mutex in Monitor correctly serializes any concurrent writes (e.g., from a cmd.Cancel callback racing with GetHistory) so there is no data race. The new test exercises the full path with a real subprocess and checks both the error category and the surfaced output text.

No files require special attention.

Important Files Changed

Filename	Overview
internal/process/process_command.go	Adds processLogger.Clear() before cmd.Start() and enriches prematureExit() with GetHistory(); ordering and synchronization are correct because cmdDone is closed only after cmd.Wait() (which drains all I/O), so GetHistory() is always called after all subprocess writes have completed.
internal/process/process_command_test.go	Adds TestProcessCommand_PrematureExitSurfacesOutput, which passes an unknown flag to the simple-responder binary and verifies both the premature-exit error text and that the upstream flag error appears in the returned error message.

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