feat: add legacy V1 setup detection for self-update compatibility (US-036)

[sven1103-agent]

Summary

Implements US-036 - Self-update preserves legacy V1 setups.

Changes

• detect_legacy_setup(): New function to identify V1 bundled-preset projects by checking for manifest presence and helper_version < 2.0.0
• show_migration_guidance(): New function that outputs actionable migration steps to stderr
• cmd_validate: Modified to warn about legacy setup but not fail (exits 0)
• cmd_preset_current: Modified to show legacy indicator in output

Acceptance Criteria Met

• ✅ Given an existing V1 helper installation, when I run opencode-helper self-update and the latest available version is V2+-capable, then the CLI upgrades successfully using the existing self-update workflow.
• ✅ Given a project that uses a legacy bundled-preset setup, when I run the upgraded V2+ CLI, then the project remains usable without immediate migration.
• ✅ Given the upgraded V2+ CLI detects a legacy bundled-preset setup, when I run a relevant setup or status command, then the CLI reports that the project is using legacy setup state and provides actionable migration guidance.

Technical Notes

• Legacy detection checks: manifest presence, config-sources.json (V2 indicator), helper_version field
• Migration guidance is non-blocking per REQ-F-030
• No project files are modified during self-update