feat(adapters): Slack Block Kit rendering for agent_milestone events

[krokoko]

Context: ROADMAP.md → Channels → Slack notification polish (milestone portion) + What's ready notification plane follow-up
Related: #112 (approval_requested buttons—separate), smart-progress-updates draft

---

Component

API or orchestration

Describe the feature

Replace generic fallback text in Slack fanout for unmapped event types—especially agent_milestone—with rich Block Kit sections (phase, milestone name, cost/turn, link). When smart progress updates ship, render structured done / next / blockers fields in-thread.

Use case

Roadmap notes shipped Slack dispatcher uses generic fallback for many milestones. Operators in Slack cannot scan progress meaningfully compared to GitHub edit-in-place comments.

Proposed solution

1. Extend slack-notify.ts with milestone-specific Block Kit templates per allowlisted milestone (pr_created, nudge_acknowledged, etc.).
2. Map progress_report events when smart progress lands.
3. Respect --verbose / per-workspace filters from INTERACTIVE_AGENTS.md AD-4.
4. Terminal dedup and threaded reply behavior unchanged.
5. Golden tests for Block Kit payload shapes.

Other information

• Add Slack-button approvals for Cedar HITL approval gates #112 covers in-thread approve/deny for approval_requested; this issue covers milestone/progress rendering.

• What's ready (line 80): "richer milestone and approval-gate rendering is follow-up work."

• Design context: docs/design/INTERACTIVE_AGENTS.md, cdk/src/handlers/slack-notify.ts.

• This might be a breaking change

[krokoko]

Complements #112 (approval buttons). Should render #487 smart progress when that ships.