feat: iterate-spec should reply to review comments + post per-iteration changelog

## Problem

`iterate-spec` reads inline review comments, top-level PR comments, and reviews, then updates artefacts and commits. The only visible output is the sticky status comment:

> ✅ spec updated by openspec-flow

The reviewer has no visible mapping between what they asked for and what the bot did. Inline review threads stay unanswered and unresolved.

## Gaps

1. **No inline reply.** Each unresolved inline review comment should get a bot reply (`Done in ` / `Addressed — see :` / `Won't fix — `) so the thread shows resolution.
2. **No thread resolution.** Where the bot fully addressed a thread, mark it resolved via GraphQL `resolveReviewThread`.
3. **No iteration changelog.** Sticky status comment (or a one-shot reply on the PR) should list:
- `@alice asked for multi-line handling → updated proposal §3, added task 4.2`
- `@bob asked to tighten failure contract → revised design.md error table`
So a scroll-through reader sees the round-trip without diffing.

## Out of scope

- Replying on the originating issue's comments (issue is read-only context for the bot during iterate-spec).
- Replying on `create-spec` — first-pass spec PR has nothing to acknowledge yet.
- `iterate-impl` — same gap exists; track separately or expand scope here.

## Notes

- Bot must keep ignoring its own comments to avoid feedback loops.
- Best-effort: a failed reply/resolve should warn-log, never block the commit/push.

Start at the iterate-spec entry point and trace how inline review comments, top-level PR comments, reviews, and the sticky status comment are handled. Implement best-effort replies, GraphQL thread resolution, and a per-iteration changelog while preserving self-comment filtering; done means failures warn-log without blocking commit or push.

Written by the indexing model from the issue text.