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>

commands/gsd/execute-phase.md

@@ -99,7 +99,6 @@ During execution, handle discoveries automatically:
 2. **Auto-add critical** - Security/correctness gaps, add and document
 3. **Auto-fix blockers** - Can't proceed without fix, do it and document
 4. **Ask about architectural** - Major structural changes, stop and ask user
-5. **Log enhancements** - Nice-to-haves, log to ISSUES.md, continue
 
 Only rule 4 requires user intervention.
 </deviation_rules>