docs: Add plugin-side daemon spawn pattern documentation (#1056)

[unn-Known1]

Summary

This PR addresses issue #1056 by adding comprehensive documentation for plugin-side daemon spawning.

Changes

1. New Documentation: docs/developer/plugin-daemons.md

Added a complete guide for plugin authors on how to spawn background daemons, including:

• Verified spawn pattern from src/browser/bridge.ts:132-137 with detailed explanation
• Key properties: detached: true, stdio: "ignore", .unref(), and environment inheritance
• Why the pattern works: No setsid or explicit process group management needed on macOS/Linux
• Lazy spawning: Spawn from CLI entry point, not at module import
• Daemon communication: HTTP server patterns and port selection
• Multi-daemon considerations: Unique ports, discovery, conflict handling
• Testing guidelines: Verification checklist for plugin daemon authors

2. Updated: docs/superpowers/specs/2026-03-31-daemon-lifecycle-redesign.md

Added Multi-Daemon Namespace Reservation section that:

• Defines daemon naming convention (e.g., browser-bridge for built-in)
• Reserves CLI surface for opencli daemon status/stop/restart [name]
• Specifies status response format for both single and multi-daemon scenarios
• Proposes discovery mechanism via ~/.opencli/daemons/<name>.json
• Includes backward compatibility notes

Issue Resolution

• Add a short "plugin-side daemons" section to developer docs citing the spawn pattern
• Amend the daemon lifecycle redesign spec to reserve single-vs-multi daemon namespace in the CLI surface

Testing

Documentation changes only. Verified:

• New file follows existing docs style and structure
• Links to related documentation are valid
• Code examples match actual implementation in src/browser/bridge.ts

Screenshots

N/A - Documentation changes only