file-pr: Open a Concise Why-First PR

# File PR Before filing, check whether a PR for this branch already exists. Review the diff locally against the base branch (usually `origin/main`) to make sure its contents match the goal. PR titles usually become commit messages, so follow the repository's title conventions. Look at recently merged PRs and Git history for examples. Prefer a concise, human-readable title that explains why the change matters: BAD > ❌ perf(server): negotiate permessage-deflate on the websocket GOOD > ✅ perf(server): cut websocket frame size by 70%+ with gzipping Open the description with a simple explanation of the problem based on the user's original prompt, then briefly explain the solution. Do not lead with an implementation inventory: BAD > ❌ Removed implicit workspace carry-over from every "new thread" entry point (cmd+n / cmd+shift+o, sidebar v1/v2 buttons, command palette). New threads inherit only the project from context; branch, worktree, and env mode always come from the configured defaults. Deleted buildContextualThreadOptions, startNewThreadInProjectFromContext, and the v1 sidebar's seed-context machinery. GOOD > ✅ My "new worktree" default was ignored when starting new threads on existing worktrees. Super unintuitive. Now your preferences always apply. End the description with the public model name and harness that made the changes, for example `Created with <model> in <harness>.` Include subagent models that wrote part of the change. Use the model name the harness reports. If model identity is unavailable, name the harness without guessing a model. Before submission, check the title, description, quotes, and attachments for secrets and private details. Open a real PR rather than a draft so review bots run. If `gh pr create` fails because the GraphQL rate limit is used up, open it through REST with `gh api repos/<owner>/<repo>/pulls -f title=... -f head=... -f base=... -F body=@<file>`. If the user also asked to babysit it, continue with the `babysit-pr` skill. Do not post a follow-up PR comment unless it serves the user's request. When a comment is needed, use the `leaving-pr-comments` skill.