A failed run's feed post names the pull or issue it was for and its first error line, and links to that line of the log #5812

Right now a failed run posts a builder run failed: <summary> and links to the run. The owner cannot tell what broke without opening it.

Done when:

• [ ] The run.finished post with state failed (src/feed/items.ts) reads a builder run on #N failed: <first error line, 120 chars>. The error line comes from the run's stored log (the first line matching the check's error/fail markers). When there is none, it falls back to the summary.
• [ ] The post links to the run's log at that line, using an anchor the run page already supports or one this pull adds.
• [ ] Reading the error line costs no full log read on every home render. It is captured when the run finishes and kept with the event.
• [ ] A test finishes a failed run with a known log and checks the words and the link, on / and in MCP feed.