OSS launch polish: fix hook paths, add OSS files, update README

- Fix slashes in agent identity causing invalid hook file paths (gt-vqhc)
- Add Prerequisites section to README (gt-vzic)
- Create CONTRIBUTING.md, CODE_OF_CONDUCT.md, SECURITY.md (gt-xbfw)
- Update Install section for future package managers (gt-7wcf)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -15,12 +15,31 @@ Beads        = Railroad Tracks (the persistent ledger)
 
 The goal is a "village" architecture - not rigid hierarchy, but distributed awareness where agents can help neighbors when something is stuck. Whether this actually works at scale is something we're still discovering.
 
+## Prerequisites
+
+- **Go 1.23+** - For building from source
+- **Git** - For rig management and beads sync
+- **tmux** - Required for agent sessions (all workers run in tmux panes)
+- **Claude Code CLI** - Required for agents (`claude` command must be available)
+
 ## Install
 
+**From source (recommended for now):**
+
 ```bash
 go install github.com/steveyegge/gastown/cmd/gt@latest
 ```
 
+**Package managers (coming soon):**
+
+```bash
+# Homebrew (macOS/Linux)
+brew install gastown
+
+# npm (cross-platform)
+npm install -g @anthropic/gastown
+```
+
 ## Quick Start
 
 ```bash