refactor(agents): remove interrogator, merge design Q&A into conductor

Confirmed via official Copilot CLI docs: there is no ask_user/elicitation
tool available to custom agents (only execute/read/edit/search/agent/web/
todo aliases exist). Subagent calls made through the 'agent' tool are
stateless — they run to completion and return one final result, with no
mechanism to pause mid-task for a live human reply.

This means conductor invoking interrogator via the agent tool could never
work: interrogator would run as a subagent regardless of whether conductor
itself was foreground or backgrounded, and subagents can't get real user
answers. That's why it was silently writing fabricated decisions.md/
questionnaire.md content instead of actually asking anything.

Fix: delete the interrogator agent entirely and fold its full Q&A
procedure directly into conductor's own DESIGN PHASE, run inline in
conductor's own foreground turn — never delegated. Updated the design
skill/prompt, AGENTS.md, README, QUICKSTART, and role-tier config to
match. Single entry point, no subagent path for anything that needs a
live human answer.

AGENTS.md

@@ -16,7 +16,7 @@ We never reimplement an agent runtime. If the host can do it, the host does it.
 ## Pipeline
 
 ```
-design  → gated Q&A, nothing inferred        → .agents/specs/<slug>/decisions.md
+design  → conductor asks inline, user answers → .agents/specs/<slug>/decisions.md
 plan    → spec + acceptance criteria + DAG   → spec.md, plan.dag.json
 build   → worktree lanes, Ralph loop         → builder ⇄ verifier
 integrate → sequential merge + full verify   → integrator
@@ -31,8 +31,7 @@ planned around.
 
 | Agent | Tier | Invokable | Responsibility |
 | --- | --- | --- | --- |
-| `conductor` | strong | yes | Entry point. Owns the pipeline and the dynamic DAG. |
-| `interrogator` | strong | yes | Design-phase Q&A with recommended answers. |
+| `conductor` | strong | yes | Single entry point. Asks design questions itself, then owns the pipeline and the dynamic DAG. |
 | `scout` | cheap | no | Read-only recon, fanned out N-wide. |
 | `architect` | strong | yes | Spec, acceptance criteria, architecture doc, ADRs. |
 | `splitter` | mid | no | Decomposes spec into lanes with file-scope globs. |