chore: remove the Greptile review configuration (#83)

## Related Issue

No issue — housekeeping. The problem is described below.

## Problem

Greptile's trial credits are exhausted. Both of its "reviews" on #82 are
the same notice:

> `elkaix` has reached the 50-credit limit for trial accounts.

Its check still reports **success**, so a PR looks reviewed by two bots
when only CodeRabbit read it. A review bot that passes without reviewing
is worse than no bot, because the green check is misleading.

## What changed

Removes `.greptile/` (`config.json`, `files.json`, `rules.md`).
CodeRabbit is unaffected and remains the review bot.

The GitHub App installation is separate from this configuration and is
removed through the organization's settings — deleting these files alone
does not uninstall it.

## Verification

Configuration-only change: no source, tests, or build inputs are
touched. `git show --stat` is three deletions under `.greptile/`.

## Checklist

- [x] I have read the CONTRIBUTING document.
- [x] I have explained the problem above.
- [ ] I have added tests that prove my feature works. — not applicable;
removes bot configuration only.
- [x] Ran `gen-changesets` skill, or this PR needs no changeset. — no
changeset: nothing published changes.
- [x] Ran `gen-docs` skill, or this PR needs no doc update.

Author: elkaix