docs: promote stub .adoc to faithful AsciiDoc from canonical .md (Item 11 Group A) (#159)

These CONTRIBUTING docs had a 407-byte placeholder .adoc and the real
content living only in .md — the inverse of normal estate drift.
Resolution: convert the canonical .md into the .adoc (vocab-preservation
gated, >=98% coverage) and remove the now-redundant .md, so the estate
AsciiDoc-canonical convention holds without losing content.

Promoted: robot-repo-automaton/CONTRIBUTING (vocab_coverage=100% missing=0 OK)

Co-authored-by: hyperpolymath <hyperpolymath@users.noreply.github.com>
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: hyperpolymath

robot-repo-automaton/CONTRIBUTING.adoc

@@ -1,20 +1,120 @@
 // SPDX-License-Identifier: PMPL-1.0-or-later
-= Contributing Guide
+= Clone the repository
+git clone https://github.com/hyperpolymath/robot-repo-automaton.git
+cd robot-repo-automaton
 
-== Getting Started
+= Using Nix (recommended for reproducibility)
+nix develop
 
-1. Fork the repository
-2. Create a feature branch from `main`
-3. Sign off commits (`git commit -s`)
-4. Submit a pull request
+= Or using toolbox/distrobox
+toolbox create robot-repo-automaton-dev
+toolbox enter robot-repo-automaton-dev
+= Install dependencies manually
 
-== Commit Guidelines
+= Verify setup
+just check   # or: cargo check / mix compile / etc.
+just test    # Run test suite
+[listing]
+----
 
-* Conventional commits: `type(scope): description`
-* Sign all commits (DCO required)
-* Atomic, focused commits
+### Repository Structure
+----
+robot-repo-automaton/
+├── src/                 # Source code (Perimeter 1-2)
+├── lib/                 # Library code (Perimeter 1-2)
+├── extensions/          # Extensions (Perimeter 2)
+├── plugins/             # Plugins (Perimeter 2)
+├── tools/               # Tooling (Perimeter 2)
+├── docs/                # Documentation (Perimeter 3)
+│   ├── architecture/    # ADRs, specs (Perimeter 2)
+│   └── proposals/       # RFCs (Perimeter 3)
+├── examples/            # Examples (Perimeter 3)
+├── spec/                # Spec tests (Perimeter 3)
+├── tests/               # Test suite (Perimeter 2-3)
+├── .well-known/         # Protocol files (Perimeter 1-3)
+├── .github/             # GitHub config (Perimeter 1)
+│   ├── ISSUE_TEMPLATE/
+│   └── workflows/
+├── CHANGELOG.md
+├── CODE_OF_CONDUCT.md
+├── CONTRIBUTING.md      # This file
+├── GOVERNANCE.md
+├── LICENSE
+├── MAINTAINERS.md
+├── README.adoc
+├── SECURITY.md
+├── flake.nix            # Nix flake (Perimeter 1)
+└── Justfile             # Task runner (Perimeter 1)
+[listing]
+----
 
-== License
+---
 
-Contributions licensed under project license.
+## How to Contribute
 
+### Reporting Bugs
+
+**Before reporting**:
+1. Search existing issues
+2. Check if it's already fixed in `main`
+3. Determine which perimeter the bug affects
+
+**When reporting**:
+
+Use the [bug report template](.github/ISSUE_TEMPLATE/bug_report.md) and include:
+
+- Clear, descriptive title
+- Environment details (OS, versions, toolchain)
+- Steps to reproduce
+- Expected vs actual behaviour
+- Logs, screenshots, or minimal reproduction
+
+### Suggesting Features
+
+**Before suggesting**:
+1. Check the [roadmap](ROADMAP.md) if available
+2. Search existing issues and discussions
+3. Consider which perimeter the feature belongs to
+
+**When suggesting**:
+
+Use the [feature request template](.github/ISSUE_TEMPLATE/feature_request.md) and include:
+
+- Problem statement (what pain point does this solve?)
+- Proposed solution
+- Alternatives considered
+- Which perimeter this affects
+
+### Your First Contribution
+
+Look for issues labelled:
+
+- [`good first issue`](https://github.com/hyperpolymath/robot-repo-automaton/labels/good%20first%20issue) — Simple Perimeter 3 tasks
+- [`help wanted`](https://github.com/hyperpolymath/robot-repo-automaton/labels/help%20wanted) — Community help needed
+- [`documentation`](https://github.com/hyperpolymath/robot-repo-automaton/labels/documentation) — Docs improvements
+- [`perimeter-3`](https://github.com/hyperpolymath/robot-repo-automaton/labels/perimeter-3) — Community sandbox scope
+
+---
+
+## Development Workflow
+
+### Branch Naming
+----
+docs/short-description       # Documentation (P3)
+test/what-added              # Test additions (P3)
+feat/short-description       # New features (P2)
+fix/issue-number-description # Bug fixes (P2)
+refactor/what-changed        # Code improvements (P2)
+security/what-fixed          # Security fixes (P1-2)
+[listing]
+----
+
+### Commit Messages
+
+We follow [Conventional Commits](https://www.conventionalcommits.org/):
+----
+<type>(<scope>): <description>
+
+[optional body]
+
+[optional footer]