#6 feed.atom (50, commits+releases)

Context: Agents poll N mirrors; feed must be static and valid. Tasks:

• feed.atom at build: 50 entries max, commits + releases mixed newest-first; entry = title, id (sha/version-based urn), updated RFC3339, link, escaped content summary.
• Hand-rolled XML writer (no rss crate); validate with xmllint --noout in test if available else structural asserts. Success rubric:
• Feed parses as XML; entry count <= 50; newest first (assert on fixture with known dates); release entries present once m3 lands (stub-tolerant: commits-only passes now, test updated in m3). Swarm: Owlette. Branch m2/feed.