fix: prevent readStdin() from hanging indefinitely on Windows

[myselfsiddharth]

Summary

Fixes #25

On Windows, piped stdin never emits the end event for hook subprocesses spawned by Claude Code. readStdin() in src/lib/stdin.js waited for that event with no timeout, so every hook (SessionStart, UserPromptSubmit, PreToolUse, Stop) hung at its very first line and burned the full hook timeout before any authentication or API work began.

Changes to readStdin()

• Eager parse on each chunk: hooks receive exactly one JSON object on stdin, so the promise now resolves as soon as the accumulated input parses as valid JSON. On Windows this means hooks get their input in milliseconds instead of hanging, with no reliance on the timeout at all.
• 3 second timeout fallback: if no complete input ever arrives, it resolves with {} instead of hanging. All four hook callers already handle empty input gracefully (they fall back to process.cwd() and skip work that needs session_id or transcript_path).
• Release stdin on settle: pause() plus unref() so the still-open pipe cannot keep the Node process alive after the promise settles, which is the underlying mechanism of the hang.

Unix behavior is unchanged: end still resolves normally, and malformed JSON with a closed stdin still rejects with the same error message as before.

Bundles in plugin/scripts/ were rebuilt with npm run build (the bundle diff churn is from esbuild output formatting; the only source change is src/lib/stdin.js).

Test plan

Verified on Windows 11 by spawning child processes against both the source module and the built bundles:

Scenario	Result
Valid JSON, stdin closed (normal Unix path)	resolved in ~10 ms
Valid JSON, stdin never closed (the Windows bug)	resolved in ~10 ms
No input, stdin never closed	resolved with {} after 3 s
Invalid JSON, stdin closed	rejects with the same parse error as before
JSON split across chunks, stdin never closed	resolved once the object completed
Built recall-hook.cjs with stdin held open	process exited in ~200 ms with correct hook output

npx biome check and npm run build both pass.

• Confirm hooks still behave normally on macOS and Linux

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