feat: add --search flag to articles command

[hanw39]

Context\nAgents need to find articles relevant to a specific topic. Scanning all articles is inefficient.\n\n## Proposed Change\nAdd --search <keyword> flag to blogwatcher articles. Matches against article title (case-insensitive substring). Combinable with --blog, --category, --all, --format json.\n\nExample: blogwatcher articles --search golang --format json