feat(sigil-hook): Antigravity install is static-posture-only by default (#112)

[Ju571nK]

What

Makes sigil-hook install --agent antigravity honest about runtime hooks. On current Antigravity (agy >= 1.0.7) the agy plugin install PreToolUse command-hook does not fire (hardware-verified, #112), so the previous behavior — silently writing a plugin bundle and invoking agy — gave false assurance of runtime protection.

New behavior:

• Default: static-posture-only. install --agent antigravity writes nothing, never invokes agy, and prints an honest message explaining that Sigil covers Antigravity via static configuration-posture scanning instead.
• --force gate. The legacy bundle-install path is preserved behind a new --force flag (for a future agy that restores command-hooks). It warns that it may still be a no-op.
• --agent antigravity --enforce is rejected (exit code 2): there is no deny path to register on current agy.

No behavior change to the static AI Guard parser, the runtime adapter, or other agents — those edits are comment/preview-string honesty only.

Why

Hardware verification on real agy 1.0.7/1.0.8: agy plugin validate reports hooks: skipped (not found), plugin list shows components: null, and an always-deny probe hook fired 0 times while the shell command ran normally. agy's hook subsystem is proto/SDK-internal, not an external command-hook plugin component. Full finding recorded on #112.

Tests

• Unit: disposition gates on --force; honest-message invariant (NOT installed / no files written / #112 / --force / static).
• Integration (tests/install_antigravity_static.rs, HOME-isolated + fake agy): no---force writes no bundle and never invokes agy; --force writes the bundle and calls agy; --enforce exits 2.
• cargo fmt clean · clippy -D warnings clean · cargo test -p sigil-hook green.

Follow-up

A separate issue tracks refreshing the Antigravity static parser's expected values (toolPermission valid set / sandbox_mode) against agy 1.0.8.

Closes #112

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