docs(agents): add AGENTS.md for AI agent context

Provides comprehensive project documentation including:
- Overview of Stream Sprout RTMP restreaming tool
- Tech stack and build instructions
- Code style and linting requirements
- Project structure and configuration details
- Commit guidelines and security considerations

Signed-off-by: Martin Wimpress <martin@wimpress.org>

AGENTS.md

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+# AGENTS.md
+
+## Overview
+
+Stream Sprout is a bash-based RTMP restreaming tool that forwards a single video source (from OBS Studio or similar) to multiple destinations like Twitch, YouTube, Owncast, and Peertube simultaneously. It uses FFmpeg's tee muxer to copy streams without transcoding.
+
+## Tech Stack
+
+- **Language:** Bash 5.0+ (single script: `stream-sprout`)
+- **Runtime dependency:** FFmpeg (RTMP server and restreaming)
+- **Configuration:** YAML parsed via awk/sed
+- **Packaging:** Nix flake, Debian .deb, Snap, Docker/Podman
+
+## Build and Run Commands
+
+```bash
+# Run directly (requires ffmpeg, bash 5.0+, awk, grep, sed)
+./stream-sprout --config stream-sprout.yaml
+
+# Show version and FFmpeg info
+./stream-sprout --version
+
+# Show system info (useful for bug reports)
+./stream-sprout --info
+
+# Nix build
+nix build
+
+# Enter development shell with all dependencies
+nix develop
+
+# Docker build and run
+docker build -t stream-sprout .
+docker run -p 1935:1935 -it -v $PWD:/data stream-sprout --config /data/stream-sprout.yaml
+```
+
+## Linting
+
+ShellCheck is enforced via CI on all pull requests.
+
+```bash
+# Run locally before committing
+shellcheck stream-sprout
+```
+
+The script includes `# shellcheck disable=SC2154` for variables set dynamically via `eval` from YAML parsing.
+
+## Code Style
+
+- Bash scripts use `#!/usr/bin/env bash`
+- Functions use `function name() {}` syntax
+- Use `local` for function-scoped variables
+- Use `readonly` for constants
+- Validation with informative error messages using Unicode icons and ANSI colours
+- Version is tracked in the script: `readonly VERSION="x.y.z"`
+
+## Project Structure
+
+```
+stream-sprout           # Main bash script (single file)
+stream-sprout.yaml      # Local config (gitignored)
+stream-sprout.yaml.example  # Example configuration
+package.nix             # Nix package definition
+devshell.nix            # Nix development shell
+flake.nix               # Nix flake
+Dockerfile              # Alpine-based container
+```
+
+## Configuration
+
+YAML config with two main sections:
+
+- `server:` - RTMP server settings (ip, port, app, key, archive options)
+- `services:` - Destination services (each with enabled, rtmp_server, key)
+
+Config search order: `./stream-sprout.yaml`, `$XDG_CONFIG_HOME/stream-sprout.yaml`, `/etc/stream-sprout.yaml`
+
+## PR and Commit Guidelines
+
+- **Commit messages must follow [Conventional Commits](https://www.conventionalcommits.org/)**
+- PR titles are validated against Conventional Commits format
+- Single-commit PRs must have matching PR title and commit message
+- ShellCheck must pass with no warnings
+
+Common prefixes: `feat:`, `fix:`, `chore:`, `refactor:`, `docs:`
+
+## Version Updates
+
+When changing version:
+
+1. Update `VERSION` in `stream-sprout` script
+2. The Nix package extracts version automatically from the script
+
+## Constraints
+
+- Requires bash 5.0 or newer
+- FFmpeg must be available on PATH
+- RTMP only (no RTMPS support currently)
+- FFmpeg does not enforce stream keys (documented security limitation)
+
+## Security Considerations
+
+- Stream keys are stored in plain text in YAML config
+- FFmpeg accepts any RTMP stream on the configured port regardless of app/key path
+- Do not expose the RTMP port to untrusted networks without additional protection (VPN, firewall, SSH tunnel)