feat: Node.js compatibility (remove Bun runtime requirement)

[joelhooks]

Problem

The CLI requires Bun to run (#!/usr/bin/env bun) but this isn't obvious until you try to use it. Users who install via npm install -g hit env: bun: No such file or directory with no guidance.

Reported in #116 by @gaearon.

Current Bun Dependencies

Heavy usage throughout the codebase:

Bun API	Usage	Node Equivalent
Bun.$\cmd``	Shell commands (tagged template)	execa or child_process.exec
Bun.spawn()	Process spawning	child_process.spawn
Bun.file().exists()	File existence check	fs.existsSync
Bun.file().text()	Read file	fs.readFileSync
Bun.write()	Write file	fs.writeFileSync
Bun.argv	CLI arguments	process.argv
Bun.sleep()	Async delay	setTimeout promisified

Files with Bun APIs:

• bin/swarm.ts - CLI entrypoint
• src/swarm-worktree.ts - Heavy Bun.$ usage for git
• src/swarm-verify.ts - Bun.file, Bun.$
• src/swarm-decompose.ts - Bun.$
• src/tool-availability.ts - Bun.$, Bun.spawn
• src/agent-mail.ts - Bun.spawn, Bun.file
• src/hive.ts - Bun.spawn
• src/storage.ts - Bun.spawn
• src/skills.ts - Bun.spawn

Proposed Solution

Option A: Runtime Abstraction Layer (Recommended)

Create src/runtime.ts that exports platform-agnostic helpers:

// src/runtime.ts
export const runtime = {
  spawn: async (cmd: string[], opts?: SpawnOpts) => { ... },
  shell: async (cmd: string) => { ... },
  fileExists: async (path: string) => { ... },
  readFile: async (path: string) => { ... },
  writeFile: async (path: string, content: string) => { ... },
  sleep: async (ms: number) => { ... },
  argv: process.argv,
};

Detect runtime at startup, use Bun APIs when available, fall back to Node equivalents.

Option B: Compile to JS

Use bun build to compile to standalone JS. Problem: Bun.$ shell syntax won't survive compilation.

Option C: Require tsx

Change shebang to #!/usr/bin/env tsx and replace Bun APIs with Node equivalents. Requires tsx as a dependency.

Acceptance Criteria

• npm install -g opencode-swarm-plugin && swarm setup works without Bun
• All existing functionality preserved
• Tests pass on both Bun and Node
• CI tests both runtimes
• README updated with Node as primary, Bun as optional

Complexity

Estimate: Medium-High (2-3 days)

Most work is mechanical replacement, but needs careful testing of shell command edge cases.

References

• Doesn't work without bun #116 - Original report
• Bun Shell docs
• execa - Node shell alternative