fix(docs): list the contributing guide where it actually lives (#74)

The "Repository Structure" block still listed `CONTRIBUTING.md` at the
repository root, but the guide lives at `.github/CONTRIBUTING.md` — the
entry pointed at a location it had already moved away from.

This is the only change: the entry moves under the tree's existing
`.github/` branch, at the block's own comment column. The tree already
rendered correctly and its layout is untouched.

---

The contributing guide was instantiated from the estate template and
kept one of the template's tokens verbatim: `{{MAIN_BRANCH}}`, in

> 2. Check if it's already fixed in `{{MAIN_BRANCH}}`

The token is correct in the template itself, but a reader of this
repository sees it literally. It resolves to this repository's default
branch. One token — nothing else in the guide changes.

Author: hyperpolymath

.github/CONTRIBUTING.md

@@ -39,11 +39,11 @@ rattlescript/
 │   └── contractiles/    # Policy contracts (k9, dust, intend, must, trust)
 ├── .well-known/         # Protocol files (Perimeter 1-3)
 ├── .github/             # GitHub config (Perimeter 1)
+│   ├── CONTRIBUTING.md  # This file
 │   ├── ISSUE_TEMPLATE/
 │   └── workflows/
 ├── CHANGELOG.md
 ├── CODE_OF_CONDUCT.md
-├── CONTRIBUTING.md      # This file
 ├── GOVERNANCE.md
 ├── LICENSE
 ├── MAINTAINERS.md
@@ -62,7 +62,7 @@ rattlescript/
 
 **Before reporting**:
 1. Search existing issues
-2. Check if it's already fixed in `{{MAIN_BRANCH}}`
+2. Check if it's already fixed in `main`
 3. Determine which perimeter the bug affects
 
 **When reporting**: