fix: prevent statusline example script from hanging on blocked stdin

[akunzai]

Summary

• examples/statusline/statusline.sh reads stdin unbounded (jq reads directly from the live pipe). During auth/OAuth refresh or resume-conversation warm-up, the CLI's statusline runner can hold stdin open without writing or closing it, so the script hangs until the runner's ~5-10s hard kill (statusline: command failed: signal: killed). Enough consecutive kills auto-disables the custom statusline.
• Added a short timed stdin read up front (run_with_timeout 0.25 cat, with a portable fallback when GNU timeout isn't available) and close stdin immediately after (exec 0</dev/null) so nothing later in the script can block on it.
• Default JSON_INPUT to "{}" when nothing arrived in time, so the script still renders a degraded status line instead of crashing on an empty read.
• Verified with a deterministic repro (named pipe whose writer never writes/closes, mirroring the runner's documented behavior): before the fix the script hangs until an external timeout 5 kills it (exit 124, ~5s); after the fix it returns in ~0.27s with a valid degraded render (exit 0). Normal JSON input on stdin still renders exactly as before.

Closes #698

Test plan

• bash -n examples/statusline/statusline.sh
• Normal JSON input on stdin renders identical output to before the change
• Blocked-stdin repro (named pipe, writer never writes/closes) no longer hangs — returns in ~0.27s instead of being killed at the 5s test-harness timeout
• Empty/no stdin input renders a degraded status line (exit 0) instead of crashing

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[weby-homelab]

Hi @google-antigravity team and contributors! 👋

Great catch and fix in this PR regarding stdin handling in statusline.sh!

We have independently analyzed this issue and confirmed that when the CLI runner holds stdin open during OAuth refresh, authentication flow, or conversation warm-up without sending data or EOF, unbounded reads (cat or direct jq input) block indefinitely until the runner's ~5–10s hard SIGKILL limit. As noted, consecutive kills cause the CLI to auto-disable custom statusline plugins.

We have integrated this exact fix and enhanced resilience across both Bash (statusline.sh) and PowerShell (statusline.ps1) in our dedicated plugin repository:
👉 weby-homelab/antigravity-cli-statusline (v0.2.3 release)

Key Improvements in v0.2.3:

• Bash (statusline.sh): Implemented a 0.25s run_with_timeout stdin guard with exec 0</dev/null and fallback default INPUT_JSON="{}" to ensure graceful degraded rendering (~350ms response) instead of process termination.
• PowerShell (statusline.ps1): Added an asynchronous [System.Threading.Tasks.Task]::Run read with a 250ms timeout when IsInputRedirected is true.
• Empirical Validation: Verified with a non-closing named pipe reproducer; execution returns clean degraded telemetry in <360ms with exit code 0.

If anyone is looking for a production-ready, feature-rich statusline plugin with dynamic line-packing, system resource telemetry, and built-in stdin hang protection, feel free to try antigravity-cli-statusline!

Kudos to the author for initiating this fix! 🚀