feat(preamble): T3 — jargon dedup + terse-build flag (Phase A.2 + A.3)

A.2 jargon dedup: generate-writing-style.ts replaces the inlined 80-term
jargon list with a one-line pointer to scripts/jargon-list.json. The list
was duplicated into every tier-2+ skill (48 of 51 skills); inlining cost
was ~1.5 KB × 48 = ~70 KB across the corpus. Pointer cost is ~30 bytes per
skill. Agents Read the JSON once per session on first jargon term
encountered; thereafter the terms array is the canonical reference.

A.3 terse build flag: --explain-level=terse compresses preamble prose at
gen time. When the flag is set, writing-style collapses to a one-line
terse directive and completeness-section + confusion-protocol +
context-health are dropped entirely. The default build keeps the
runtime-conditional behavior intact (sections still render; the model
skips them when EXPLAIN_LEVEL: terse appears in the preamble echo). Terse
build is opt-in for users who want shipped skills to match their runtime
preference and avoid the per-session terse-mode dead prose.

TemplateContext gains an optional `explainLevel: 'default' | 'terse'`
field. Default builds set it to 'default'; --explain-level=terse sets
'terse'. Resolvers gate their output via `ctx?.explainLevel === 'terse'`.

Measured impact (default build, post-T3):
- Total corpus: 2,847 KB → 2,812 KB (saved 35 KB)
- ship.md: 160 → 159 KB
- plan-ceo-review.md: 128 → 127 KB
- Top 10 heaviest: all slightly smaller from jargon pointer

Larger compression lands in T4 (catalog trim) and T7 (atomic regen across
the full Phase A pipeline). The terse build path further compresses to
~711K tokens vs default ~725K (saved ~14K tokens corpus-wide).

Test plan:
- bun test test/gen-skill-docs.test.ts: 389 pass (no regression)
- bun test test/resolver-entry.test.ts: 6 pass
- bun test test/helpers/capture-parity-baseline.test.ts: 4 pass
- bun run gen:skill-docs --explain-level=terse: ship.md drops completeness +
  confusion-protocol + context-health sections; writing-style collapses to
  one-line terse directive

48 SKILL.md files updated (every tier-2+ skill picks up the jargon pointer).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/gen-skill-docs.ts

@@ -59,6 +59,24 @@ const MODEL_ARG_VAL: Model = (() => {
   return resolved;
 })();
 
+// ─── Explain-level Overlay ──────────────────────────────────
+// --explain-level=terse compresses preamble prose (writing-style, completeness,
+// confusion-protocol, context-health) to a single pointer line at gen time.
+// Default keeps the runtime-conditional behavior (sections render unconditionally,
+// the model skips them when EXPLAIN_LEVEL: terse appears in the preamble echo).
+// Opt-in via the build flag so most users get the runtime-flexible default.
+const EXPLAIN_LEVEL_ARG = process.argv.find(a => a.startsWith('--explain-level'));
+const EXPLAIN_LEVEL: 'default' | 'terse' = (() => {
+  if (!EXPLAIN_LEVEL_ARG) return 'default';
+  const val = EXPLAIN_LEVEL_ARG.includes('=')
+    ? EXPLAIN_LEVEL_ARG.split('=')[1]
+    : process.argv[process.argv.indexOf(EXPLAIN_LEVEL_ARG) + 1];
+  if (val !== 'default' && val !== 'terse') {
+    throw new Error(`Unknown explain level: ${val}. Use 'default' or 'terse'.`);
+  }
+  return val;
+})();
+
 // HostPaths, HOST_PATHS, and TemplateContext imported from ./resolvers/types (line 7-8)
 // Design constants (AI_SLOP_BLACKLIST, OPENAI_HARD_REJECTIONS, OPENAI_LITMUS_CHECKS)
 // live in ./resolvers/constants and are consumed by resolvers directly.
@@ -430,7 +448,7 @@ function processTemplate(tmplPath: string, host: Host = 'claude'): { outputPath:
   const interactiveMatch = tmplContent.match(/^interactive:\s*(true|false)\s*$/m);
   const interactive = interactiveMatch ? interactiveMatch[1] === 'true' : undefined;
 
-  const ctx: TemplateContext = { skillName, tmplPath, benefitsFrom, host, paths: HOST_PATHS[host], preambleTier, model: MODEL_ARG_VAL, interactive };
+  const ctx: TemplateContext = { skillName, tmplPath, benefitsFrom, host, paths: HOST_PATHS[host], preambleTier, model: MODEL_ARG_VAL, interactive, explainLevel: EXPLAIN_LEVEL };
 
   // Replace placeholders (supports parameterized: {{NAME:arg1:arg2}})
   // Config-driven: suppressedResolvers return empty string for this host