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>

get-shit-done/workflows/create-roadmap.md

@@ -130,7 +130,7 @@ Use decimal phases (2.1, 2.2) for urgent insertions:
 - Urgent work that can't wait for next milestone
 - Critical bugs blocking progress
 - Security patches needing immediate attention
-- NOT for scope creep or "nice to haves" (those go in ISSUES.md)
+- NOT for scope creep or "nice to haves" (capture with /gsd:add-todo instead)
 
 **Phase execution order:**
 Numeric sort: 1 → 1.1 → 1.2 → 2 → 2.1 → 3
@@ -368,7 +368,7 @@ Recent decisions affecting current work:
 
 (None yet)
 
-### Deferred Issues
+### Pending Todos
 
 None yet.