refactor: extract shared dep parser to lib/parse-deps.py (Closes #20)

Single source of truth for dependency parsing, replacing three copies:
- dev-poll.sh get_deps() now calls parse-deps.py
- supervisor P3b/P3c import parse_deps() via importlib

Supports stdin, argument, and --json modes for different callers.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/parse-deps.py

@@ -0,0 +1,59 @@
+#!/usr/bin/env python3
+"""Extract dependency issue numbers from an issue body.
+
+Usage:
+    echo "$ISSUE_BODY" | python3 lib/parse-deps.py
+    python3 lib/parse-deps.py "$ISSUE_BODY"
+    python3 lib/parse-deps.py --json < issues.json
+
+Modes:
+    stdin/arg:  reads a single issue body, prints one dep number per line
+    --json:     reads a JSON array of issues from stdin, prints JSON
+                dep graph: {"issue_num": [dep1, dep2], ...}
+
+Matches the same logic as dev-poll.sh get_deps():
+  - Sections: ## Dependencies / ## Depends on / ## Blocked by
+  - Inline: "depends on #NNN" / "blocked by #NNN" anywhere
+  - Ignores: ## Related (safe for sibling cross-references)
+"""
+import json
+import re
+import sys
+
+
+def parse_deps(body):
+    """Return sorted list of unique dependency issue numbers from an issue body."""
+    deps = set()
+    in_section = False
+    for line in (body or "").split("\n"):
+        if re.match(r"^##?\s*(Depends on|Blocked by|Dependencies)", line, re.IGNORECASE):
+            in_section = True
+            continue
+        if in_section and re.match(r"^##?\s", line):
+            in_section = False
+        if in_section:
+            deps.update(int(m) for m in re.findall(r"#(\d+)", line))
+        if re.search(r"(depends on|blocked by)", line, re.IGNORECASE):
+            deps.update(int(m) for m in re.findall(r"#(\d+)", line))
+    return sorted(deps)
+
+
+if __name__ == "__main__":
+    if len(sys.argv) > 1 and sys.argv[1] == "--json":
+        issues = json.load(sys.stdin)
+        graph = {}
+        for issue in issues:
+            num = issue["number"]
+            deps = parse_deps(issue.get("body", ""))
+            deps = [d for d in deps if d != num]
+            if deps:
+                graph[num] = deps
+        json.dump(graph, sys.stdout)
+        print()
+    else:
+        if len(sys.argv) > 1:
+            body = sys.argv[1]
+        else:
+            body = sys.stdin.read()
+        for dep in parse_deps(body):
+            print(dep)