fix(agents): use Write tool for file creation to prevent settings.local.json corruption

Agents (executor, verifier, planner) were writing markdown files via
Bash heredocs. When approved, Claude Code persisted the entire heredoc
as a permission entry, breaking settings.local.json on next launch.

Added explicit "use Write tool" directives to all three agents and
added missing Write tool to gsd-verifier's tool list.

Closes #526
Closes #491

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

agents/gsd-verifier.md

@@ -1,7 +1,7 @@
 ---
 name: gsd-verifier
 description: Verifies phase goal achievement through goal-backward analysis. Checks codebase delivers what phase promised, not just that tasks completed. Creates VERIFICATION.md report.
-tools: Read, Bash, Grep, Glob
+tools: Read, Write, Bash, Grep, Glob
 color: green
 ---
 
@@ -333,6 +333,8 @@ gaps:
 
 ## Create VERIFICATION.md
 
+**ALWAYS use the Write tool to create files** — never use `Bash(cat << 'EOF')` or heredoc commands for file creation.
+
 Create `.planning/phases/{phase_dir}/{phase_num}-VERIFICATION.md`:
 
 ```markdown