Fix Windows SessionEnd hang and taskkill false failures

[Kenshiro787]

Summary

Two Windows-specific problems in the plugin's process teardown, observed on Windows 11 with Node 26:

1. SessionEnd hook never returned. sendBrokerShutdown waited forever when the broker accepted the connection but never replied, so Claude Code reported the hook as cancelled. It now gives up after a timeout (2 s by default) and destroys the socket.
2. cancel failed although the task had been killed. terminateProcessTree treated any non-zero taskkill status as a failure. On Windows, taskkill /PID <root> /T /F enumerates the tree first and then terminates each entry; a short-lived descendant (git/cmd helpers spawned by the worker) that exits in between makes taskkill report "The operation attempted is not supported" and exit non-zero even though the root process was killed. This reproduced in 7 of 8 controlled runs and made the cancel integration test flaky (0/5 before the change, 3/3 after).

Changes

• broker-lifecycle.mjs: sendBrokerShutdown(endpoint, timeoutMs = 2000) resolves on timeout and clears the timer on data/error/close.
• process.mjs: on Windows, terminateProcessTree probes the root with process.kill(pid, 0) before running taskkill. A root that is already gone returns { attempted: false, delivered: false, method: null } without invoking taskkill. After a non-zero taskkill status, the root's liveness decides the outcome: root gone means delivered, root still alive keeps the previous behaviour and throws. The English-only looksLikeMissingProcessMessage filter is removed, so the result no longer depends on the localized taskkill message text (a French "introuvable" used to fall through and be reported as a successful kill). Comments state that delivered describes the root only, like the process-group SIGTERM on other platforms; it does not prove that every descendant is gone. Non-Windows code paths and the ENOENT fallback are unchanged.
• broker-endpoint.mjs: the unix endpoint path uses path.posix.join, so it stays a valid socket path when computed on Windows.
• Test fixtures on Windows: skip the broken-symlink case when the runner cannot create symlinks (no Developer Mode), use a node.cmd shim instead of a symlink, and propagate USERPROFILE next to HOME.

Tests

• New: sendBrokerShutdown times out against a mute broker; terminateProcessTree with localized taskkill output and liveness probes, taskkill skipped when the root is already absent, EPERM on the probe not treated as absent, ENOENT fallback preserved, non-Windows path unchanged. Against the previous process.mjs, 4 of the 8 process tests fail.
• Full suite on Windows 11 / Node 26.1.0, run from native cmd.exe: 98 tests, 97 passed, 0 failed, 1 skipped (the symlink fixture without Developer Mode). The cancel integration test passed 3/3 consecutive runs.

Known limitations

• delivered is root-scoped. A descendant that taskkill could not stop after the root died is not detected, which matches the process-group signal semantics on other platforms; none of the callers reads the return value.
• Narrow TOCTOU windows remain (root exiting on its own between the probe and taskkill; PID reuse before the second probe). They only affect the reported status, not the teardown itself.

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