feat(commands): add /test-pr — prepare an open PR for local human testing

[robercano-ghbot]

What

Adds a project-agnostic /test-pr <pr-number> command and its prepare-pr.sh helper.

Given a PR number it:

1. resolves the PR's head branch and fetches the latest pushed commit;
2. creates/refreshes a detached git worktree at humanTest.worktreeDir/pr-<n> (default .worktrees/pr-<n>) — isolated from the main checkout and from any agent worktree already on that branch;
3. runs humanTest.prepare (install/build) inside it and prints humanTest.launch for the reviewer to run.

New .claude/gates.json convention (humanTest.prepare / launch / worktreeDir) makes it project-agnostic; example added to examples/ts-solidity-foundry/gates.json and documented in docs/USAGE.md.

Why

Reviewers testing bot PRs from their main working tree run code that does not contain the unmerged change, conclude "the fix doesn't work", and bounce the PR — a real loop we just hit. This gives a one-shot, stale-proof way to run the actual PR branch, at the exact pushed SHA, in isolation.

Notes

• Idempotent: re-running on the same PR fast-forwards the worktree and rebuilds.
• Never edits the PR, never touches main. Detached checkout avoids git's "branch already checked out" error.
• Validated end-to-end against a real PR in a downstream project (reDeploy): worktree lands at the PR head SHA, deps build, studio dev server launches.

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