Enhance documentation with contributing guidelines, planning docs, and agent policies

.github/copilot-instructions.md

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+# Copilot Instructions for FeDIY
+
+## Mission Context
+
+FeDIY is intended to become a federated (ActivityPub) DIY project-hosting platform in Rust, inspired by communities like Ravelry and Instructables.
+
+## Hard Constraint
+
+Do not generate executable core project code.
+
+This includes Rust code and other executable logic under:
+
+- `src/`
+- `tests/`
+- Any runtime script or automation that becomes part of application behavior
+
+## Allowed Help
+
+You may assist with:
+
+- Documentation
+- Planning and architecture discussions
+- Housekeeping and project management tasks
+- Environment/tooling updates (including Nix files such as `flake.nix`, `default.nix`, and `shell.nix`)
+
+## Delivery Process Defaults
+
+- Use Red-Green TDD/BDD framing for planning and guidance.
+- Assume GitHub Flow for branching and PR lifecycle.
+- Support behavior-first test planning and review checklists.
+
+## If Asked for Code
+
+When a coding request is made, provide:
+
+1. Links to primary sources (start with https://doc.rust-lang.org/book/).
+2. A concise, paraphrased overview of the relevant concepts.
+3. A step-by-step implementation checklist for a human developer.
+
+Do not include code snippets or patches for executable core logic.