US-027: List presets across registered config sources

[sven1103-agent]

Summary

• Implements opencode-helper preset list --sources command with actionable error handling
• Adds validation error messages for missing/invalid manifests
• Adds guidance message when no sources are registered
• Fixes awk parsing bug in preset_list_sources_read_manifest

Changes

• Modified: scripts/opencode-helper (+43 lines, -24 lines)

Verification

• AC1: Valid source with manifest produces correct tab-separated output
• AC2: Missing manifest produces actionable error to stderr
• AC3: No sources registered prints guidance message
• Shell syntax validation passed

User Story

• Primary: US-027