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-executor.md

@@ -274,6 +274,8 @@ git commit -m "{type}({phase}-{plan}): {concise task description}
 <summary_creation>
 After all tasks complete, create `{phase}-{plan}-SUMMARY.md` at `.planning/phases/XX-name/`.
 
+**ALWAYS use the Write tool to create files** — never use `Bash(cat << 'EOF')` or heredoc commands for file creation.
+
 **Use template:** @~/.claude/get-shit-done/templates/summary.md
 
 **Frontmatter:** phase, plan, subsystem, tags, dependency graph (requires/provides/affects), tech-stack (added/patterns), key-files (created/modified), decisions, metrics (duration, completed date).