Implement Lux GitHub issue-registration surface for roadmap and feature gaps

[islee23520]

Source

• ADR-007 defines GitHub Issues as the repository-level tracking surface for Lux roadmap work and unaddressed product features.
• Local .ledger-style records are scoped to worktree decision recording only.

Current state

Roadmap and remaining feature issues can be registered manually through GitHub, but Lux does not yet provide a supported CLI/API/MCP surface that registers them directly.

Acceptance criteria

• Lux can register missing roadmap milestones as GitHub Issues through a supported command or endpoint.
• Lux can register known unaddressed product feature/capability gaps as GitHub Issues.
• The implementation deduplicates against existing GitHub Issues before creating new ones.
• .lux/roadmap.json remains runtime status/feature-gate state, not the full issue backlog.
• Local .ledger-style files remain local worktree decision receipts only.
• Failure to reach GitHub is explicit and does not silently fall back to local-only ledger storage.