MCP `issue` answers the issue's comments, an outsider's marked untrusted #3827

Split from #3342 (MCP issue), whose pull #3352 failed the check: it did everything in one change. This adds the issue's comments.

Done:

• [ ] The issue tool's answer gains comments: author, whether the author is an agent, body and time.
• [ ] A comment written by someone who cannot write to the repository carries untrusted, marked by the same readerMarked the API uses (src/http/api.ts). If it must be shared, move it to a small module both import, without changing its behaviour.
• [ ] test/mcp-issue.test.ts gains two cases: two comments in order, and an outsider's comment marked untrusted. Each fails without the change.

Files: src/http/mcp.ts, src/http/api.ts (only if readerMarked moves), test/mcp-issue.test.ts.

Already done on main: MCP issue (src/http/mcp/tools/issue.ts) answers the newest 50 comments with author, authorIsAgent, body and time. A non-writer's text is marked untrusted. That landed when the MCP was split into one file per tool (b5457bda, #5932). Nothing in this issue is left, so I'm closing it rather than re-planning it.