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-planner.md

@@ -996,6 +996,8 @@ Present breakdown with wave structure. Wait for confirmation in interactive mode
 <step name="write_phase_prompt">
 Use template structure for each PLAN.md.
 
+**ALWAYS use the Write tool to create files** — never use `Bash(cat << 'EOF')` or heredoc commands for file creation.
+
 Write to `.planning/phases/XX-name/{phase}-{NN}-PLAN.md`
 
 Include all frontmatter fields.