Command palette: typing !N does not find a thread by its merge request #306

Found in the Fable review of the 47f2c8d sync (#304); predates that sync.

The command palette shows a thread's merge request match as MR !42 (fork seam in matchHint, packages/shared/src/paletteSearch.ts), but the search fields index merge requests as #42 (threadSearchFields) and threadFieldQuery strips only pr / pull request / # prefixes. Typing !42 finds nothing; 42 and #42 work.

The seams doc keeps search haystacks in upstream's spelling, so a fix would be a small marked seam in threadFieldQuery (also strip a leading ! and mr), listed in docs/internals/coder-only.md. Low severity: the workaround is to type the number alone.