fix: Windows compatibility — skip /dev/tty, guard jq dependency

[ytzhao]

• Add _is_windows() helper to detect MSYS/MINGW/Cygwin environments
• Skip /dev/tty writes on Windows (no such device), always use terminalSequence JSON path instead
• Add early-exit guard in build-payload.sh when jq is missing, with a clear error message instead of confusing 'command not found'
• Add tests for _is_windows, Windows emit_terminal_sequence fallback, and jq-missing graceful degradation
• Update README with platform-specific jq install instructions

[maycuatroi1]

Repro on Windows 11 + Git Bash (Claude Code, warp plugin 2.2.0). Both failures this PR targets do happen:

build-payload.sh: line 49: jq: command not found
PostToolUse:TaskCreate hook error

After installing jq, the second one surfaces:

emit-terminal-sequence.sh: line 82: /dev/tty: No such device or address

The on-session-start.sh guard does fire, but it only suggests brew install jq / apt install jq, so on Windows there is no actionable next step. Putting the guard in build-payload.sh as this PR does also covers the other hook entry points, which currently have no guard and fail on every tool call.

Workaround in the meantime: drop jq.exe into ~/bin, Git Bash prepends that to PATH so no restart is needed.