agent-team: wrap superseded spec/plan/review blocks in <details> tags on the next iteration

[verkyyi]

Summary

After 2–3 kickbacks the issue body is a wall of spec + plan + 3× review blocks, all with the same section names. Humans scanning for "what's the current spec?" have to eyeball-diff identical headings. GitHub's <details> is the simple fix; it just isn't applied.

Proposal

Each stage, before posting its own new block, iterates over earlier <!-- agent-team:<kind> iteration=N --> blocks in the issue and wraps them via the edit-comment safe output:

<details>
<summary>Spec (iteration 1 — superseded)</summary>

[existing content]

</details>

Acceptance

• Planner collapses prior agent-team:spec blocks with iteration < current.
• Implementer collapses prior agent-team:plan blocks.
• Reviewer collapses prior agent-team:review blocks.
• Latest block stays expanded; earliest <summary> shows iteration number + "superseded".