fix(docs): list the contributing guide where it actually lives (#54)

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

@@ -1,5 +1,5 @@
 # Clone the repository
-git clone https://{{FORGE}}/hyperpolymath/patch-bridge.git
+git clone https://github.com/hyperpolymath/patch-bridge.git
 cd patch-bridge
 
 # Using Nix (recommended for reproducibility)
@@ -35,11 +35,11 @@ patch-bridge/
 │   └── contractiles/    # Policy contracts (k9, dust, lust, 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
@@ -58,7 +58,7 @@ patch-bridge/
 
 **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**:
@@ -91,10 +91,10 @@ Use the [feature request template](.github/ISSUE_TEMPLATE/feature_request.md) an
 
 Look for issues labelled:
 
-- [`good first issue`](https://{{FORGE}}/hyperpolymath/patch-bridge/labels/good%20first%20issue) — Simple Perimeter 3 tasks
-- [`help wanted`](https://{{FORGE}}/hyperpolymath/patch-bridge/labels/help%20wanted) — Community help needed
-- [`documentation`](https://{{FORGE}}/hyperpolymath/patch-bridge/labels/documentation) — Docs improvements
-- [`perimeter-3`](https://{{FORGE}}/hyperpolymath/patch-bridge/labels/perimeter-3) — Community sandbox scope
+- [`good first issue`](https://github.com/hyperpolymath/patch-bridge/labels/good%20first%20issue) — Simple Perimeter 3 tasks
+- [`help wanted`](https://github.com/hyperpolymath/patch-bridge/labels/help%20wanted) — Community help needed
+- [`documentation`](https://github.com/hyperpolymath/patch-bridge/labels/documentation) — Docs improvements
+- [`perimeter-3`](https://github.com/hyperpolymath/patch-bridge/labels/perimeter-3) — Community sandbox scope
 
 ---