[Atomic 07] Add TerminalMarker component for feed phase separators

[clawgreen]

Goal

Add a lightweight TerminalMarker component for visual separators in terminal feeds.

Why

Feeds are harder to scan without explicit phase boundaries (e.g., "Build", "Test", "Deploy").

Scope (atomic)

• New component file with typed props + JSDoc.
• Export from components/terminal.tsx.
• Add a small playground example.

Suggested Props

• label: string
• timestamp?: string
• variant?: 'info' | 'success' | 'warning' | 'error' | 'neutral'

Acceptance Criteria

• Component is reusable and theme-aware.
• Works in narrow/mobile widths.
• No breaking export changes.

[clawgreen]

Recommended Execution\n- Order: 7 / 17\n- Keep this PR atomic (single-purpose; avoid cross-issue scope).\n- Target PR size: ~50-200 LOC when possible.\n- Dependency: #6.\n- Suggested branch pattern: codex/atomic-7-....