feat: Add documentation files including Code of Conduct, Contributing guidelines, and Security policy; update Markdown linting configuration and scripts

CONTRIBUTING.md

@@ -0,0 +1,101 @@
+# Contributing to EchoHub
+
+Thanks for your interest in contributing — EchoHub aims to stay lightweight, terminal-first, and self-hosted.
+
+## Quick start (dev)
+
+### Prerequisites
+
+- .NET 10 SDK
+
+### Build
+
+```bash
+dotnet build src/EchoHub.slnx
+```
+
+### Test
+
+```bash
+dotnet test src/EchoHub.slnx
+```
+
+### Run (local)
+
+Server:
+
+```bash
+dotnet run --project src/EchoHub.Server
+```
+
+Client:
+
+```bash
+dotnet run --project src/EchoHub.Client
+```
+
+## What to work on
+
+- Check open issues (especially `good first issue` / `help wanted` if present)
+- Docs fixes in `docs/` are always welcome
+- Tests: `src/EchoHub.Tests/`
+
+If you’re proposing a larger change, open an issue first so we can align on approach.
+
+## Code style & expectations
+
+- Keep PRs focused (small and reviewable)
+- Prefer clear naming over cleverness
+- Add/adjust tests for bug fixes when it’s practical
+- Avoid committing secrets (JWT secrets, tokens, connection strings)
+
+## Docs
+
+This repo uses DocFX for the site in `docs/`.
+
+To build docs locally you typically need the assemblies built in Release first:
+
+```bash
+dotnet build src/EchoHub.slnx --configuration Release
+```
+
+Then run DocFX:
+
+```bash
+docfx docs/docfx.json
+```
+
+## Markdown lint
+
+CI lints Markdown. Locally:
+
+- On Linux/macOS (or Windows with Git Bash/WSL):
+
+```bash
+./scripts/lint-markdown.sh
+```
+
+- Anywhere with Node.js installed:
+
+```bash
+npx --yes markdownlint-cli2
+```
+
+## Pull requests
+
+- Fill out the PR template
+- Ensure `dotnet test src/EchoHub.slnx` is green
+- Mention any behavioral changes (client UX, auth, uploads)
+
+## Commit messages
+
+Any consistent style is fine; descriptive subjects help reviews.
+Examples:
+
+- `fix(server): validate image magic bytes`
+- `feat(client): add /servers improvements`
+- `docs: clarify getting started`
+
+## Reporting security issues
+
+Please do **not** file public issues for security problems. See `SECURITY.md`.