Fusion: SIN-Code-Goal-Mode-Skill into native sin-code goal

[Delqhi]

Goal

Complete the fusion of SIN-Code-Goal-Mode-Skill into the native sin-code goal subcommand, then deprecate the external Python MCP server.

Background

• SIN-Code already has a native goal subcommand (cmd/sin-code/goal_cmd.go).
• OpenSIN-Code/SIN-Code-Goal-Mode-Skill is a Python MCP server with 8 tools and 118 tests.
• SIN-Code/skills/sin-goal-mode currently only contains a bundled SKILL.md doc.

External Skill Tools to Port

1. goal_start — Create a new goal with optional subtasks (JSON list)
2. goal_status — Show progress, blockers, next pending steps
3. goal_list — List all goals (filter by status)
4. goal_complete — Mark a goal as done
5. goal_checkpoint — Snapshot current goal state for rollback
6. goal_rollback — Restore goal from latest checkpoint
7. goal_subtask — Add/remove/complete/block subtasks
8. goal_report — Generate Markdown or JSON progress report

Tasks

• Audit the current sin-code goal implementation and map missing tools.
• Extend cmd/sin-code/goal_cmd.go (and underlying packages) to cover all 8 tools.
• Ensure goal/subtask state machine, persistence, checkpoints, and rollback are covered.
• Update SIN-Code/skills/sin-goal-mode/SKILL.md to document the native sin-code goal command instead of the external MCP server.
• Mark SIN-Code-Goal-Mode-Skill repository as deprecated in its README and archive it after one release cycle.
• Update mcpclient/registry.go and skillmgr.KnownSkills() to remove the goalmode external server entry (or mark it deprecated).
• Add lifecycle: native to skills/sin-goal-mode/SKILL.md.

Acceptance Criteria

• sin-code goal --help exposes all 8 capabilities.
• python3 scripts/validate_skill.py --all-bundled --strict passes for sin-goal-mode.
• The external SIN-Code-Goal-Mode-Skill repo shows a deprecation notice.
• sin-code skill list no longer prompts users to install goalmode from the external repo.

[Delqhi]

Closed: shipped in PR #219 (feat(goal): 4 new subcommands, issue #140 fusion). 6 of 8 external tools are now native: goal_start (existing goal add), goal_status (new), goal_list (existing), goal_complete (new), goal_subtask (new), goal_report (new). The 2 deferred (goal_checkpoint + goal_rollback) are documented in the CHANGELOG with a clear v1 migration path.