plan(agent): consolidate core runtime roadmap

[fffonion]

Summary

• Consolidates the stack-added roadmap documents into one canonical Agent-focused core plan.
• Records the seven-layer rewrite, capability ownership, unused-surface audit, verification matrix, and rollback procedure.
• Marks WebSocket, generic RSS task scheduling, wide-local bytecode, and unrelated backend-convergence work outside the current Agent production scope.

Stack

• Base: master
• Next: refactor(compiler): build semantic module graph #15

Verification

• Plan-only commit
• Exactly one stack-added plan relative to master
• git diff --check passed

Supersedes

Planning content from #22 and #25, plus the stack-added standalone plan files folded into this roadmap.