fix(comments): disallow posting threads GitHub cannot show

[moritzwilksch]

GitHub accepts a review comment on a file the PR does not change, or on a line past its three context lines, and then shows it nowhere. diffle let such threads into a pending review. Now a thread GitHub cannot show carries a githubBlocker: its Add button and the sidebar's Add all are disabled with the reason, and buildReview skips it and reports why.

Per-thread blocker. CommentStore derives githubBlocker next to stale, at relocation and import, from a ReviewView that now exposes inDiff and a side's onGithub ranges. buildReview skips by the field alone; the client only reads it.

 export const CommentThreadSchema = z.object({
   stale: z.boolean(),
   staleFromLine: z.number().optional(),
+  githubBlocker: z.string().optional(),
 });

 export interface SideView {
   shown: LineRange[] | null;
+  onGithub: LineRange[];
 }
 export interface ReviewView {
   hasFile(path: string): boolean;
+  inDiff(path: string): boolean;
 }

Narrowing to a context. shownRanges(patch, side, context) keeps the lines within context of a change, as a diff generated with that context would show them. GITHUB_CONTEXT_LINES = 3 feeds onGithub.

Two bugs on the way. GitRepo pins diff.suppressBlankEmpty=false: with it on, an empty context line loses its leading space, and the hunk-body walk overran into the next hunk header and dropped later hunks. Import now places a line by the same shows predicate as relocation, so a line the diff does not show gets a 400 instead of going stale on the next refresh.

Open decision. Lines reached by expanding context or the full-file view of a changed file are outside the hunks, so commenting there now fails immediately; before, it was accepted and went stale on the next refresh. Allowing such comments instead, fresh with a GitHub blocker, is a product call.

Closes #168

Screenshots

Fixture: main..feature with a real GitHub PR mocked via gh, one open thread inside the hunk (L5–6) and one three lines past GitHub's context (L12) or on a file outside the diff (README.md).

Before	After