docs(adr): revise ADR-0063 to two agents bound by surface; add ADR-0064 tool scoping (#2167)

Reverses ADR-0040's unified single-assistant core (merged first cut of
ADR-0063 in #2164 kept it). ask and build are two products for different
needs — like Claude Chat vs Claude Code — bound by the surface the user is
in, not a per-turn intent classifier or a roster. Keeps ADR-0040's "user
never picks", re-grounded as surface binding.

- ADR-0063 (revised): two agents (ask ≈ Chat, free/OSS; build ≈ Code,
  paid/cloud); skills (+tools/MCP) are the only third-party extension
  primitive; *.agent.ts closed to third parties; skill→agent surface
  affinity; ask/build canonical naming; supersedes ADR-0040 core + §3.
- ADR-0064 (new): tool scoping — an agent's tools = the union of its
  skills' tools; remove the global fall-through; resolves ADR-0040 §4.
- Assessment: four-repo touchpoint inventory + findings that drove D2.

Co-authored-by: Jack Zhuang <277994282+os-zhuang@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: 3 people