v0.4.0: Improved agent setup & CLI output

Added

• Added agent harness setup for Claude, Codex, Cursor, and GitHub Copilot via revpack setup agent <target>.

Changed

• Changed revpack checkout to prepare the review bundle by default, so checkout now leaves the review context ready to use.
• Improved incremental review instructions so agents focus on newly changed code and threads while still allowing important findings outside the incremental diff.
• Improved revpack status output with clearer sections, better next-step guidance, and a stale bundle indicator when the prepared bundle no longer matches the latest PR/MR head.
• Improved publish command output and follow-up guidance.
• Reworked README and command documentation for the current setup and review workflows.

Fixed

• Improved checkout support on Windows by automatically using core.longpaths=true for git commands.
• Fixed resolved threads appearing in the incremental "changed threads since last checkpoint" context section.
• Fixed GitHub bot-user detection so bot comments are not incorrectly treated as human review comments or findings.
• Fixed unresolved thread flags so comments from other bots such as CodeRabbit are not labeled as revpack SELF threads.

Deprecated

• Deprecated revpack setup --prompts. Use revpack setup agent copilot for the Copilot prompt.
• Deprecated revpack checkout --prepare. Checkout prepares by default now, and the flag is kept only for compatibility.