fix(cycle): extract_atoms stamps concepts so synthesize_concepts has material (#2123) (#2124) (#3308)

synthesize-concepts.ts's design comment says extract_atoms stamps a
`concepts:` frontmatter field on each atom and :92 consumes ONLY that
field — but the extractor never wrote it, so the atoms → concepts
pipeline was dead end-to-end: every cycle reported "synthesize_concepts:
skipped — no atoms with concept refs" no matter how many atoms
accumulated (696 page-derived atoms / 0 with concepts on our production
brain before an external backfill).

Fix, all on the extractor side (no synthesize change needed):
- EXTRACT_PROMPT asks for `concepts` (1-3 kebab-case TOPIC labels) with
  an explicit reuse-over-coinage instruction — labels must cluster,
  since synthesize_concepts only materializes groups of >=2.
- parseAtomsResponse validates labels (kebab regex, max 3, drop
  invalid; empty -> undefined).
- The putPage frontmatter write stamps `concepts` alongside lesson /
  source_quote.

Tests: 4 parse cases + an end-to-end regression that goes extractor ->
real frontmatter -> synthesize_concepts' OWN DB query path -> concept
page. The existing tests fed synthesize via the `_atoms` seam, which is
exactly how this gap survived.

Validated in production ahead of this PR by stamping the same shape
externally: the next synthesize_concepts run wrote 33 concept pages
(T2=7/T3=26) from 60 stamped atoms, zero failures.

Co-authored-by: 陈源泉 <84364275+ChenyqThu@users.noreply.github.com>
Co-authored-by: 陈源泉 <chenyuanquan@chenyuanquandeMac-mini.local>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

src/core/cycle/extract-atoms.ts

@@ -171,10 +171,20 @@ interface ExtractedAtom {
   body: string;
   source_quote?: string;
   lesson?: string;
+  /**
+   * 1-3 kebab-case topic labels for concept clustering. Consumed by
+   * synthesize_concepts (groups atoms by `frontmatter.concepts`; only
+   * labels shared by >=2 atoms materialize a concept page, so the prompt
+   * biases reuse-over-coinage). #2123.
+   */
+  concepts?: string[];
   virality_score?: number;
   emotional_register?: string;
 }
 
+/** kebab-case validator for concept labels ("captive-portal", "channel-pricing"). */
+const CONCEPT_LABEL_RE = /^[a-z0-9]+(-[a-z0-9]+)*$/;
+
 const EXTRACT_PROMPT = `You extract atomic content nuggets from a transcript.
 
 An atom is a single-source, self-contained idea that could become a tweet,
@@ -185,12 +195,17 @@ quote, or short essay angle. Each atom must:
 
 Output a JSON array of atoms (1-3 per transcript, never more than 3).
 Each atom: {title (≤80 chars), atom_type, body (2-4 sentences),
-source_quote (verbatim ≤200 chars), lesson (one sentence), virality_score
-(0-100), emotional_register (one of: shocking, inspiring, funny, sobering,
-practical, controversial)}.
+source_quote (verbatim ≤200 chars), lesson (one sentence), concepts
+(1-3 topic labels), virality_score (0-100), emotional_register (one of:
+shocking, inspiring, funny, sobering, practical, controversial)}.
 
 atom_type MUST be one of: ${ATOM_TYPES.join(', ')}.
 
+concepts are kebab-case English TOPIC labels used to cluster atoms into
+concept pages (e.g. "captive-portal", "channel-pricing-strategy") — never
+entity or brand names. Use the same label for the same topic across atoms;
+prefer a label you already used over coining a near-synonym.
+
 Output ONLY the JSON array, no prose.`;
 
 interface DiscoveredPage {
@@ -600,6 +615,7 @@ export async function runPhaseExtractAtoms(
                 source_hash: item.contentHash.slice(0, 16),
                 ...(atom.source_quote && { source_quote: atom.source_quote }),
                 ...(atom.lesson && { lesson: atom.lesson }),
+                ...(atom.concepts && atom.concepts.length > 0 && { concepts: atom.concepts }),
                 ...(atom.virality_score !== undefined && { virality_score: atom.virality_score }),
                 ...(atom.emotional_register && { emotional_register: atom.emotional_register }),
                 extracted_at: new Date().toISOString(),
@@ -736,6 +752,13 @@ export function parseAtomsResponse(raw: string): ExtractedAtom[] {
       body,
       source_quote: typeof obj.source_quote === 'string' ? obj.source_quote.slice(0, 500) : undefined,
       lesson: typeof obj.lesson === 'string' ? obj.lesson : undefined,
+      concepts: (() => {
+        if (!Array.isArray(obj.concepts)) return undefined;
+        const labels = obj.concepts
+          .filter((c): c is string => typeof c === 'string' && CONCEPT_LABEL_RE.test(c))
+          .slice(0, 3);
+        return labels.length > 0 ? labels : undefined;
+      })(),
       virality_score:
         typeof obj.virality_score === 'number' &&
         obj.virality_score >= 0 &&