feat: add Streaming Diff Oven

[alowpoly]

What is the Streaming Diff Oven?

Streaming Diff is a local review dashboard that turns ordinary Codex edits into timestamped diff cards scoped to one Codex task.

The user opens the viewer, manually attaches that browser tab to a task feed, and continues working normally. Trusted Codex lifecycle hooks snapshot the repository around tool calls and atomically publish a thread-specific feed. The dashboard receives revisions over SSE and supports unified or split diffs.

Manual attachment lets two browser tabs follow two agents working in the same repository without attributing changes from the shared Git tree alone.

Included

• Per-task hook capture and atomic retained feeds
• Manual viewer attachment, disconnect, and task selection
• Loopback/token-gated controller endpoints and SSE updates
• Timestamped diff cards with file types, KPIs, and unified/split layouts
• Contract, hook-isolation, and server tests
• The checkout's accompanying Checklist and Differential Testing dashboard changes

Boundaries

• The Oven remains declarative and non-executable.
• The dashboard is read-only except for ephemeral viewer bindings.
• Project hooks require one-time Codex trust review and load for new or resumed tasks.

Validation

• npm run build:dashboard
• npm run verify — 175 tests passing

Review note

This draft contains the full working checkout requested for publication. The branch predates a recent upstream dashboard directory move, so it will need a focused port or conflict resolution before merge.