Rework the README hero for agent discovery

[wilbeibi]

Tunes the hero copy so LLM/agent retrieval can find and correctly classify catchup.

• h1 + tagline: lead with the category phrase ("context handoff for AI coding agents") and the action ("resume a session in another agent").
• Intro: query-shaped triggers ("usage limit", "switch tools mid-task") + concrete pairings (Claude Code→Codex, Cursor→OpenCode) that match specific questions.
• Use-it line: restores the "don't want to explain the whole job again" hook, front-loads the usage-limit trigger.
• Homebrew blurb (.goreleaser.yaml): same messaging on the brew info discovery surface.

Body copy stays minimal — one restatement of identity, one pairing clause, full Works with list untouched as the entity anchor.