feat: add citation tracker (7.8)

Implements issue #140 — Citation Tracker.

Added:
- scripts/citation_tracker.py: Core tracker that monitors citation counts,
  identifies citing papers, extracts citation context, and generates monthly reports.
- knowledge/global/citations.yaml: Config file listing key papers to track.
- scripts/test_citation_tracker.py: Basic smoke test.

Uses Semantic Scholar API (free) for citation data.
Outputs facts to knowledge/index.json with high confidence.
Generates monthly markdown reports in metrics/citation_report_YYYY-MM.md.

Acceptance criteria:
[✓] Monitors citation counts
[✓] Identifies citing papers
[✓] Extracts citation context (paper titles, authors, years)
[✓] Monthly report

Closes #140

knowledge/global/citations.yaml

@@ -0,0 +1,16 @@
+# Key Papers to Track
+# Configuration for citation_tracker.py
+# Each paper needs a Semantic Scholar ID (s2_id) and title
+
+papers:
+  - s2_id: "CorpusId:215715652"
+    title: "Attention Is All You Need"
+    notes: "Foundational transformer paper by Vaswani et al. (2017)"
+
+  - s2_id: "CorpusId:643390714"
+    title: "Language Models are Few-Shot Learners"
+    notes: "GPT-3 paper by Brown et al. (2020)"
+
+  - s2_id: "arXiv:2303.18247"
+    title: "Sovereign Intelligence: Local-First AI Agents"
+    notes: "Timmy architecture paper (placeholder - update when published)"