feat(core): allow plan mode to write/edit PLAN files exclusively - #43710

Summary

• create ~/.opencode/plan when the Plan plugin starts
• enforce the Plan directory whitelist through final agent permission rules
• keep edit, write, and patch visible while denying mutation resources outside the Plan directory
• include the absolute directory in Plan reminders and blocked-mutation messages

Testing

• bun test test/plugin/plan.test.ts
• bun typecheck
• bunx oxlint packages/core/src/plugin/plan.ts packages/core/src/plugin/internal.ts packages/core/test/plugin/plan.test.ts
• git diff --check