ADR-1235 step 1: route the trivial-converter runtime group (cursor, windsurf, augment, trae, codebuddy) off the inline install() agent loop onto the descriptor-driven installRuntimeArtifacts path. Establishes the converter-context foundation (pre-converter cross-cutting + no agent-stamp). Agent install output is byte-identical for all 16 runtimes (golden-parity, global + verified local). cline deliberately excluded (local rules-only). Closes#1763.
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