Antigravity was the last agent posting hook status through Windows PowerShell 5.1. Every
hook event — roughly one every 2-6s during an active session — paid a ~300ms interpreter
cold start, which is what made the console the agent allocates for each hook last long
enough to be seen as continuous flashing.
Move the Windows POST to the shared curl.exe builder every other agent already uses, via
the `extraFormLines` escape hatch for the `hook_event_name` field Antigravity uniquely
needs. Measured on Windows 11: 326ms -> 134ms per event.
Because the curl line percent-expands its arguments, the script also needs
`setlocal DisableDelayedExpansion` (#9358/#9941) so a `!` in a pane key or worktree path
is not eaten as a delayed reference.
curl omits a `--data-urlencode name@-` field entirely when stdin is empty, so accept an
absent or blank Antigravity payload as `{}` at the ingest boundary — the POSIX script
substitutes `{}` before posting and PowerShell did the same, and without this a
payload-less event lost the status transition its `hook_event_name` still carried. Scoped
to that source; every other agent keeps rejecting a body it cannot parse.
Adds a cross-agent guard asserting the invariant the original drift violated: a managed
Windows .cmd hook posts through fully-qualified curl.exe and spawns no interpreter.
Generated under a mocked win32 platform so the POSIX CI legs guard it too.
Validated on a real Windows 11 host, not an emulated platform check.
Fixes#15117
Neil
36d78e88af