FE-1264: Block landing in comparison runs

[kostandinang]

Why

Landing from an isolated execution comparison invalidates the attempt and can leave changes in the host workspace.

What

Comparison runtimes now omit /brunch:land and execute_land_preflight, and the landing handler refuses policy-disabled calls before inspection, confirmation, or mutation. Normal runtimes retain confirmation-gated landing.

How to test

• npm run verify:full

[cursor]

PR Summary

Medium Risk
Touches host-mutation landing paths and comparison runtime wiring; behavior is gated and well-tested, but incorrect policy defaults could block legitimate landings or leave a bypass in comparisons.

Overview
Isolated execution comparison runtimes can no longer land promoted work into the host workspace, so comparison attempts stay valid and the host is not mutated.

A new allowHostLanding policy (default on) threads through Pi extension options and execute-land registration. When it is false, registerBrunchExecuteLand skips registering /brunch:land and execute_land_preflight, and runBrunchLandCommand exits immediately with a warning—before preflight, git inspection, confirmation, or apply. Comparison launches set allowHostLanding: false alongside the existing comparisonIsolation restrictions (e.g. allowWebTools: false). Normal product runtimes are unchanged.

Tests cover policy-disabled command behavior, empty registration, and a Pi runtime integration asserting no landing command or preflight tool in comparison mode.

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