fix(ssh): self-heal daemon bootstrap uploads on remote hosts - #10590

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Greptile Summary

This PR fixes remote daemon bootstrap uploads by preserving SSH stdin before starting the asynchronous payload reader.

• Duplicates stdin to descriptor 3 for the background cat and closes the descriptor after the reader exits.
• Adds a behavioral regression test that executes the generated command with file-backed stdin and verifies the complete payload and writer-marker cleanup.
• Updates upload-command recognition in package and workspace tests.

Confidence Score: 5/5

The PR appears safe to merge with no actionable correctness or security failures identified.

The generated command preserves SSH stdin using POSIX shell redirections, retains bounded timeout, verification, and cleanup behavior, and is covered by a behavioral test using real file-backed stdin.

Important Files Changed

Sequence Diagram

sequenceDiagram
    participant Local as cmux
    participant SSH as SSH process
    participant Shell as Remote sh
    participant Cat as Background cat
    participant Temp as Temporary payload
    Local->>SSH: Start command with binary as stdin
    SSH->>Shell: sh -c upload script
    Shell->>Shell: "exec 3<&0"
    Shell->>Cat: "cat <&3 (background)"
    Cat->>Temp: Write complete payload
    Shell->>Cat: wait
    Shell->>Shell: "exec 3<&-"
    Shell->>Shell: Remove writer marker
    Shell-->>Local: Exit status

_{Reviews (1): Last reviewed commit: "fix(ssh): preserve stdin for background ..." | Re-trigger Greptile}

CI note: the three failed-job retries all stop before app-host test execution on shared CMUXAuthCore / CmuxAuthRuntime undefined-symbol linker errors (the same failure is present on unrelated PR branches). The Swift package matrix also repeatedly times out in the unrelated CmuxIrohTransport CmxConnectivityPeerSessionTests suite after its built-in retry. The focused remote-daemon Go lane, runtime/warning lane, Linux preflight, and all non-app-host checks are green; no actionable failure points to this diff. Attempt 3 is still running for the final package result.

Final CI retry result (attempt 3): the focused CmuxRemoteSession package suite passed 164 tests in 27 suites, including Background upload reader consumes the SSH stdin stream; the runtime/warning lane and all Linux/non-app-host checks also passed. The aggregate workflow remains red only because (1) all four app-host shards fail before test execution on missing CMUXAuthCore/CmuxAuthRuntime linker symbols, reproduced across three fresh-runner attempts and unrelated branches, and (2) the package matrix later times out twice in the unrelated CmuxIrohTransport CmxConnectivityPeerSessionTests suite. No actionable failure involves this PR. PR remains mergeable/clean.