fix(gen): wire boundaryInstruction from host config; drop three no-op binDir ternaries

hosts/codex.ts declared boundaryInstruction and nothing read it — review.ts
kept its own byte-identical CODEX_BOUNDARY literal (verified equal + trailing
escaped newlines). The resolver now reads the config, so the boundary has one
owner. (autoplan's template carries deliberately generic variants, enforced by
gen-skill-docs.test.ts:1358 — untouched by design.)

The 'ctx.host === codex ? $GSTACK_BIN : ctx.paths.binDir' ternary appeared in
three resolvers and could never change the result: resolvers/types.ts already
sets binDir to $GSTACK_BIN for every usesEnvVars host including codex.

Proof: zero-diff regen for claude AND codex hosts; gen-skill-docs +
host-config suites green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

scripts/resolvers/preamble/generate-context-recovery.ts

@@ -1,7 +1,7 @@
 import type { TemplateContext } from '../types';
 
 export function generateContextRecovery(ctx: TemplateContext): string {
-  const binDir = ctx.host === 'codex' ? '$GSTACK_BIN' : ctx.paths.binDir;
+  const binDir = ctx.paths.binDir; // env-var hosts already resolve to $GSTACK_BIN via types.ts
 
   return `## Context Recovery