Slice 6 — User can create, list, and view Issues on a Repository

[safayavatsal]

What to build

The Issue primitive at MVP. Distinguishing feature vs. GitHub: an Issue must be assignable to an Agent (slice 7 wires the Agent path; this slice ships the data shape that supports it).

• Web UI: per-Repository Issues tab. New issue form (title, body in Markdown). Issue list (open/closed). Issue detail page with body, comments, and an "Assignee" field that at this slice accepts only humans (Agent assignment lights up in slice 7).
• platform-api: POST /repos/:o/:n/issues, GET /repos/:o/:n/issues, GET /issues/:id, POST /issues/:id/comments, POST /issues/:id/close, POST /issues/:id/reopen.
• The assignee field on an Issue is polymorphic: {kind: "user" | "agent", id: ...}. At this slice only kind: "user" is wired through; the schema accommodates agent for slice 7.

Acceptance criteria

• A user can open an Issue with a title and Markdown body.
• Issue list shows open and closed Issues separately, sorted by recency.
• Issue detail page renders Markdown safely (no XSS).
• Comments can be added, listed in order.
• Closing and reopening an Issue updates the list view immediately.
• The assignee schema supports kind: "agent" at the data layer even though the UI doesn't expose it yet.

Blocked by

• Slice 3 — User can create an empty Repository and 'git clone' it over HTTPS #3