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

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

• #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

Start with cdk/src/handlers/slack-notify.ts and the AD-4 guidance in docs/design/INTERACTIVE_AGENTS.md, then review the Slack fanout handling for unmapped events. Done means milestone and progress events use the requested Block Kit payload shapes, filters are respected, and golden tests cover them without changing terminal deduplication or threaded replies.