Refine Work Plan task boundaries

[shinpr]

Summary

• Replace file-count task sizing with semantic boundaries based on implementation outcome, rollback boundary, and executor lane.
• Make Work Planner the sole owner of task boundaries and materialize each approved Work Plan item into one task file.
• Update task templates and workflow terminology across backend, frontend, and fullstack plugins.
• Bump the bundled plugin version to 0.22.9.

Validation

• pnpm sync:check
• pnpm run check:skills-index
• git diff --check
• claude plugin validate for the marketplace and all four bundled plugins