feat(acp-client): sticky plan button and modal for chat window

[raphaeltm]

Summary

• When an agent creates a plan, it scrolls out of view as the conversation grows. This adds a persistent sticky button above the chat input that shows plan progress at a glance.
• Clicking the button opens a modal with the full plan, status indicators, and a progress bar.
• The button adapts its styling: blue/pulsing when work is in progress, green when all complete, gray when all pending.

Validation

• pnpm lint
• pnpm typecheck
• pnpm test — 227 tests pass (21 new)
• Additional validation run (if applicable)
• Mobile and desktop verification notes added for UI changes

UI Compliance Checklist (Required for UI changes)

• Mobile-first layout verified — button is compact, modal respects small screens with max-height 60vh
• Accessibility checks completed — dialog role, aria-modal, aria-label, Escape key, focus trap
• Shared UI components used or exception documented — follows ConfirmDialog modal pattern

Exceptions (If any)

N/A

Agent Preflight (Required)

• Preflight completed before code changes

Classification

• external-api-change
• cross-component-change
• business-logic-change
• public-surface-change
• docs-sync-change
• security-sensitive-change
• ui-change
• infra-change

External References

N/A: Frontend-only change within existing acp-client package. No external APIs involved.

Codebase Impact Analysis

• packages/acp-client/src/components/StickyPlanButton.tsx — New component
• packages/acp-client/src/components/PlanModal.tsx — New component
• packages/acp-client/src/components/AgentPanel.tsx — Integration (imports, state, render)
• packages/acp-client/src/index.ts — New exports

Documentation & Specs

N/A: No behavior change to existing APIs or user-facing documentation. Components are self-documented via types and tests.

Constitution & Risk Check

• Principle XI (No Hardcoded Values): No hardcoded URLs, timeouts, or limits. Modal z-index uses inline value consistent with existing patterns.
• No security implications — purely presentational components reading existing plan data from conversation items.