feat(tooling): add decision show <D-NNN> with implementing tickets + refs

Closes the decision-to-ticket coverage gap (#723). The `decisions-coverage`
Makefile target already reported per-decision counts; this adds the
single-decision drill-down via `tooling/db/decision show D-159`, returning
linked tickets, outbound refs, and inbound refs in one shot.

Schema unchanged — reverse link is a SELECT on tickets.decision_ref.

Refs: #723

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tooling/db/decisions_sync.py

@@ -414,6 +414,49 @@ def claim_id(cfg, prefix, domain, title):
         conn.close()
 
 
+def show_decision(cfg, decision_id):
+    """Show a single decision with full details including linked tickets and cross-refs."""
+    conn = get_connection(cfg)
+    try:
+        row = conn.execute(
+            "SELECT * FROM decisions WHERE id = ?",
+            (decision_id,),
+        ).fetchone()
+
+        if not row:
+            return {"ok": False, "error": f"Decision not found: {decision_id}"}
+
+        decision = dict(row)
+
+        # Implementing tickets: tickets where decision_ref = this ID
+        ticket_rows = conn.execute(
+            "SELECT id, title, status, type FROM tickets"
+            " WHERE decision_ref = ? ORDER BY id",
+            (decision_id,),
+        ).fetchall()
+        decision["implementing_tickets"] = [dict(t) for t in ticket_rows]
+
+        # Cross-refs outbound: references from this decision to others
+        refs_out = conn.execute(
+            "SELECT target_id, ref_type, note FROM decision_refs"
+            " WHERE source_id = ? ORDER BY target_id",
+            (decision_id,),
+        ).fetchall()
+        decision["refs_out"] = [dict(r) for r in refs_out]
+
+        # Cross-refs inbound: other decisions referencing this one
+        refs_in = conn.execute(
+            "SELECT source_id, ref_type, note FROM decision_refs"
+            " WHERE target_id = ? ORDER BY source_id",
+            (decision_id,),
+        ).fetchall()
+        decision["refs_in"] = [dict(r) for r in refs_in]
+
+        return {"ok": True, "decision": decision}
+    finally:
+        conn.close()
+
+
 def check_dupes(cfg):
     """Check for duplicate decision IDs across all markdown files."""
     # Pre-existing collisions too deeply embedded to renumber (139+ references).
@@ -461,6 +504,7 @@ Settled Reach Decisions Sync & ID Management
 
 Usage:
   decisions_sync.py sync                        Parse decisions/*.md and upsert into SQLite
+  decisions_sync.py show <D-NNN>                Show a decision with linked tickets + refs
   decisions_sync.py next [D|Q|R]                Show next available ID (all prefixes or one)
   decisions_sync.py claim <D|Q|R> <domain> [title]  Claim next ID and insert placeholder
   decisions_sync.py check-dupes                 Check for duplicate IDs across markdown files
@@ -493,6 +537,11 @@ def main():
 
     if cmd == "sync":
         result = sync(cfg)
+    elif cmd == "show":
+        if len(sys.argv) < 3:
+            result = {"ok": False, "error": "Usage: show <decision_id>  e.g. show D-159"}
+        else:
+            result = show_decision(cfg, sys.argv[2])
     elif cmd == "next":
         result = next_id(cfg, sys.argv[2] if len(sys.argv) > 2 else None)
     elif cmd == "claim":