feat: move stats footer into Python engine, harden synthesis voice contract

DO NOT MERGE until local validation passes on 3+ golden topics.

Problem: the magic footer (✅ All agents reported back!, emoji tree,
Top voices, Raw results saved) was composed by the synthesizer model
following a "Copy this EXACTLY" template buried 1150 lines into SKILL.md.
Under context pressure, Opus 4.7 dropped it. Three recent /last30days
runs (Opus 4.7, programming language for AI agents, Kanye West)
produced clean prose with no footer and used AI-slop section headers
(## The launch, ## Where it disappoints) instead of flowing paragraphs.

Fix:

1. render.py: new _render_emoji_footer() emits the deterministic footer
   as the final block of every compact output. Zero-count sources are
   omitted. Tree characters (├─ / └─) computed from populated-line
   count. The model no longer assembles the tree from text instructions.

2. render.py: new _site_name_for_url() and _format_web_line_sources()
   map URLs to clean publication names (Later, Buffer, CNN, etc.) so
   the 🌐 Web line is pre-assembled by Python.

3. last30days.py: compute_save_path_display() turns the save path into
   a ~/-relative string that the engine puts in the footer. Signature
   change: emit_output() and render_compact() both accept save_path.

4. SKILL.md synthesis contract rewritten:
   - Footer template DELETED. Replaced with instruction to include the
     engine footer block verbatim.
   - URL-to-site-name sub-block DELETED. Engine does this.
   - "Calculate actual totals" paragraph DELETED. Engine does this.
   - All em-dashes in the synthesis section replaced with ` - ` (single
     hyphen with spaces). Em-dashes are the most reliable AI-slop tell.
   - New rules: no ## markdown section headers in response body, no
     invented title line like "{Topic}: last 30 days", no bold section
     labels acting as headers. Bold-lead-in paragraph shape stays.
   - SELF-CHECK updated to verify footer presence, no em-dashes, no
     body-level headers.

Tests: 15 new tests covering footer emission, zero-source omission,
tree character placement, save-path threading, URL-to-name helper,
Web line formatting, Top voices combination, Polymarket line.

All 127 tests pass across render, rerank, cluster, briefing, CLI,
internals, fun-scoring.

Plan: docs/plans/2026-04-17-003-feat-deterministic-footer-plan.md

Local validation protocol (blocks merge):
- Run /last30days in a fresh Claude Code window on 5 golden topics
- Verify each output contains the footer block verbatim
- Verify zero ## body headers, zero em-dashes/en-dashes, zero invented
  title lines
- Report 5x8 pass/fail matrix; all 40 cells must be green before merge

🤖 Generated with Claude Opus 4.7 (1M context) via [Claude Code](https://claude.com/claude-code) + Compound Engineering v2.63.1

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

scripts/last30days.py

@@ -112,16 +112,36 @@ def save_output(report: schema.Report, emit: str, save_dir: str, suffix: str = "
     return out_path
 
 
-def emit_output(report: schema.Report, emit: str, fun_level: str = "medium") -> str:
+def emit_output(report: schema.Report, emit: str, fun_level: str = "medium", save_path: str | None = None) -> str:
     if emit == "json":
         return json.dumps(schema.to_dict(report), indent=2, sort_keys=True)
     if emit in {"compact", "md"}:
-        return render.render_compact(report, fun_level=fun_level)
+        return render.render_compact(report, fun_level=fun_level, save_path=save_path)
     if emit == "context":
         return render.render_context(report)
     raise SystemExit(f"Unsupported emit mode: {emit}")
 
 
+def compute_save_path_display(save_dir: str, topic: str, suffix: str, emit: str) -> str:
+    """Compute the user-friendly save path string that will be shown in the footer.
+
+    Uses ~ for the home directory so the footer reads "~/Documents/Last30Days/slug-raw.md"
+    instead of an absolute machine-local path.
+    """
+    from pathlib import Path as _Path
+    path = _Path(save_dir).expanduser().resolve()
+    slug = slugify(topic)
+    extension = "json" if emit == "json" else "md"
+    suffix_part = f"-{suffix}" if suffix else ""
+    raw = path / f"{slug}-raw{suffix_part}.{extension}"
+    try:
+        home = _Path.home().resolve()
+        relative = raw.relative_to(home)
+        return f"~/{relative}"
+    except ValueError:
+        return str(raw)
+
+
 def persist_report(report: schema.Report) -> dict[str, int]:
     import store
 
@@ -373,7 +393,12 @@ def main() -> int:
         pass
 
     fun_level = config.get("FUN_LEVEL", "medium").lower()
-    rendered = emit_output(report, args.emit, fun_level=fun_level)
+    footer_save_path = None
+    if args.save_dir:
+        footer_save_path = compute_save_path_display(
+            args.save_dir, report.topic, args.save_suffix or "", args.emit
+        )
+    rendered = emit_output(report, args.emit, fun_level=fun_level, save_path=footer_save_path)
     if args.save_dir:
         save_path = save_output(report, args.emit, args.save_dir, suffix=args.save_suffix or "")
         sys.stderr.write(f"[last30days] Saved output to {save_path}\n")