docs: ADR-015 — Inter-agent communication patterns

## Summary

Define the communication protocol between development-time agents operating in different roles (Planner, Implementor, Reviewer per ADR-009).

## Context

- ADR-009 establishes role boundaries but intentionally defers the communication mechanism
- PR #144 review identified this gap (reviewer @scoropeza, confirmed by @scottschreckengaust)
- Current state: single-agent-per-task model with no inter-agent communication

## Scope

- Event-based communication model (asynchronous, not direct messaging)
- Modalities: GitHub issue/PR comments, EventBridge events, TaskEventsTable entries
- Discovery: agents discover work via issue assignment, not peer-to-peer negotiation
- No direct agent-to-agent messaging (avoids coupling and race conditions)

## Dependencies

- ADR-009 (role boundaries — prerequisite, established)
- ADR-003 (governance — issue-based coordination)
- ADR-012 (operational knowledge stack — enforcement layer)

## Acceptance criteria

- [ ] ADR-015 draft with status `proposed`
- [ ] Communication modalities documented with pros/cons
- [ ] Interaction patterns for common multi-agent workflows (plan → implement → review)
- [ ] Amend ADR-009, ADR-003 as needed after ADR-015 is accepted