fix(post-install): honour the conditions and fields formulas declare

[indaco]

Description

A step could carry an instruction the executor did not implement and still report success. Guards were the widest case: only three step types checked them, so a copy told to run only under a condition ran unconditionally and overwrote what the formula meant to keep. Guards now gate every step type, the unless_exists condition formulas use to protect existing user data is honoured, and a step carrying a field the executor cannot honour refuses loudly instead of doing partial work under a completed banner.

Seven more formulas now finish their post-install rather than warning: arcadedb, bareos-client, cayley, kafka, opensearch, rpm and tronbyt-server.

Related Issue

• None

Notes for Reviewers

• The refusal list names what each step honours rather than what it ignores, so a field upstream adds later is loud on arrival instead of silently dropped.
• Checked against every formula in homebrew-core: the set that completes post-install grows from 104 to 111, and no formula loses one it completes today.