Issues one author opens within two minutes fold into one feed post: "planned 10 issues in trevato/forge", each one a line under it #5820

A planner run opens ten issues at once, and today each one is its own opened an issue post (src/feed/items.ts, issue.opened). That buries everything else.

Done when:

• [ ] When one author opens 3 or more issues in one repository within 2 minutes, the feed shows one post: planned N issues in owner/name. It ranks where the newest one would, and links to the repository's map.
• [ ] The first 3 titles show under the post, each linking to its issue, followed by and N more.
• [ ] One or two issues keep today's single posts.
• [ ] The chip counts and cursor paging stay correct. A test seeds 10 issues from one author and 1 from another, finds 2 posts, and pages through without a duplicate.
• [ ] MCP feed folds the same way.

Files: src/feed/feed.ts, src/feed/items.ts, tests. Risk: #5808 folds pulls in the same place. Keep the fold step one function that both can use.

Parked by the crew: the check stayed red through 2 automatic tries (pull #5840 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.