chore: remove dead ISSUES.md system

Remove the global ISSUES.md deferred enhancement tracking system.

- Delete /gsd:consider-issues command (never used)
- Delete issues.md template (never instantiated)
- Remove Rule 5 from deviation rules (never triggered)
- Remove all ISSUES.md, ISS-XXX, and "deferred issues" references
- Update STATE.md to track pending todos instead

The ISSUES.md system was designed to capture non-critical enhancements
during plan execution via "Rule 5", but it never fired in practice
across 100+ projects. The system added ~350 lines of dead code.

The /gsd:add-todo and /gsd:check-todos system serves the same purpose
and is actually used.

Note: UAT *-ISSUES.md files (per-plan, created by /gsd:verify-work)
are unaffected - those are a separate, active system.

Closes #56

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -251,7 +251,6 @@ GSD handles it for you:
 | `STATE.md` | Decisions, blockers, position — memory across sessions |
 | `PLAN.md` | Atomic task with XML structure, verification steps |
 | `SUMMARY.md` | What happened, what changed, committed to history |
-| `ISSUES.md` | Deferred enhancements tracked across sessions |
 | `todos/` | Captured ideas and tasks for later work |
 
 Size limits based on where Claude's quality degrades. Stay under, get consistent excellence.
@@ -342,7 +341,6 @@ You're never locked in. The system adapts.
 | `/gsd:list-phase-assumptions [N]` | See what Claude thinks before you correct it |
 | `/gsd:pause-work` | Create handoff file when stopping mid-phase |
 | `/gsd:resume-work` | Restore from last session |
-| `/gsd:consider-issues` | Review deferred issues, close resolved, identify urgent |
 | `/gsd:add-todo [desc]` | Capture idea or task from conversation for later |
 | `/gsd:check-todos [area]` | List pending todos, select one to work on |
 | `/gsd:debug [desc]` | Systematic debugging with persistent state across `/clear` |