fix(hooks): run managed bootstrap in Antigravity PreInvocation ensure hooks (#472)

[ZacLou]

Fixes #472

The Antigravity PreInvocation ensure hooks now call the managed bootstrap script (scripts/ensure-monk-agent.{sh,ps1}) on cold start, so a plugin upgrade refreshes monk-agent automatically instead of perpetually restarting the existing stale binary.

• �nsure-monk-agent.sh invokes the sibling bootstrap, captures only its stdout (the resolved binary path), and suppresses stderr to keep hook stdout JSON-clean.
• �nsure-monk-agent.ps1 invokes the bootstrap with the new -Quiet switch and redirects non-output streams to $null.
• The .ps1 bootstrap gains a -Quiet switch that suppresses its Write-Host progress message, ensuring only the binary path is emitted on stdout.
• Added regression tests covering the new wiring.

[NyxSpecter4]

Reporter of #472 here — verified the hook wiring: both Antigravity ensure hooks now call the managed bootstrap on the cold path (table on #472). Two review notes:

1. .antigravity-plugin/scripts/ensure-monk-agent.ps1 — param() is not the first statement. The diff adds param([switch]$Quiet) after $ErrorActionPreference = "Stop". In PowerShell a script's param block must precede all other statements (comments/#requires/using excepted); I believe this makes the script fail to parse, which would break the bootstrap for every host that calls it (not just Antigravity) — please verify on Windows PowerShell 5.1 and move param() to line 1.
2. Bootstrap inside a PreInvocation hook. On the cold path the hook now runs a checksum + possible download before the agent starts. Worth noting in the PR body what the worst-case latency is against Antigravity's hook timeout, and that a failed download falls back to the on-disk binary (the .sh side does; the .ps1 side relies on $ResolvedPath being empty — please confirm the bootstrap writes nothing to stdout on failure).

Also: the branch is based on v0.1.59; main is v0.1.60 — a rebase keeps the diff to 5 files. Happy to re-run once updated.

[ZacLou]

Thanks for the careful review @NyxSpecter4 — all three points addressed in 3366349:

1. param() ordering — moved param([switch]$Quiet) to the first statement; $ErrorActionPreference = "Stop" now sits below it with a comment explaining the parse rule. The rebase onto v0.1.60 was clean (release touches disjoint files).
2. Bootstrap latency & fallback — worst-case cold path: checksum fetch (~1 request) + 62MB archive download measured at line rate ~6.0s with progress suppressed, Expand-Archive 1.8s — total well under VSCode's 60s subprocess ceiling. On failure: .sh falls back to the on-disk binary; .ps1 writes nothing to stdout on the failure path — the hook calls it with -Quiet 2>$null 3>$null 4>$null 5>$null 6>$null, so only the resolved binary path ever reaches the hook's stdout, and the hook itself falls back to $env:MONK_AGENT_PATH or the default install location when $ResolvedPath is empty.
3. Rebase — rebased onto v0.1.60 (8d8efce); the regression suite tests/ensure-monk-agent-auto-update-472.sh passes.

Happy to iterate further if anything else looks off.