feat(agent-runtime): draft rationale from the audit pass + on-demand drafting

[kmonsoe]

Stacked on #865 (base is feat/conv-review-queue-read-model — retarget to main before merging if #865 lands first, per the usual stack discipline).

Implements handover item 4 ("reasoning is nearly free") plus the runtime half of "Ask for a draft":

• rationale in the audit contract (agent-runtime/src/audit.ts): the audit pass now returns {"rationale": "...", "actions": [...]} — one or two sentences for the human reviewer stating what the reply asserts and what grounds it. Parsed defensively (trimmed, capped at 1000 chars, null when absent); all fail-open paths return rationale: null.
• Parked on the draft: in draft mode the handler passes rationale + the turn's toolNames through setDraftReply; backend stores them on the draft_reply metadata (retrievedDocumentIds already lived there). The review pane's "why" block reads straight off the draft and hides when no rationale is present — no auditEnabled plumbing needed (it has none in production today).
• draft-request handler mode: the runner subscribes to conversation.draft_requested (emitted by POST /v1/conversations/:id/request-draft from feat(conv): review-queue read model, internal notes, draft reject stamps #865) and runs the handler with draft delivery forced — even on agentMode: auto conversations — tolerating the requester's own claim and an existing assignee. Voice/closed/no-end-user/off guards stay.
• Event plumbed through realtime-event-bus (in-process bus) and the agent-runtime realtime client, wired in agent-host/src/runner.service.ts next to greet.

Tests

• agent-runtime: 466 passed (new: rationale surfaced/capped/nulled, rationale lands in setDraftReply opts, requestDraft parks a draft on a claimed auto conversation without posting).
• backend-core conv + realtime: 88 passed (new: metadata round-trip of rationale + toolNames). agent-host: 146 passed. Typecheck clean across all three.

🤖 Generated with Claude Code

https://claude.ai/code/session_01CPwvC1MqF1uhpNqC8BmvQN