fix(review): trim comments to AGENTS.md's one/two-line why-only rule

Comments in mobile-diff-image-preview.ts and mobile-file-tab-doc.ts ran
3-6 lines and narrated mechanism instead of stating only the non-obvious
reason, per AGENTS.md's "Code Comments: Document the Why, Briefly" rule.

Co-authored-by: Orca <help@stably.ai>

mobile/src/files/mobile-diff-image-preview.ts

@@ -1,10 +1,7 @@
 import { buildImageDataUri } from '../../../src/shared/image-data-uri'
 
-// A git.diff result for a binary file. The host (git.diff) already sends base64
-// image bytes plus isImage/mimeType for previewable formats, so mobile can render
-// image diffs instead of falling back to "Binary preview unavailable". The
-// originalIsBinary/modifiedIsBinary flags tell a deletion (modified side absent)
-// apart from a modify whose bytes the host couldn't inline (relay/size-capped).
+// originalIsBinary/modifiedIsBinary distinguish a genuine deletion from a modify
+// whose bytes the host couldn't inline (relay/size-capped).
 export type MobileBinaryDiffResult = {
   kind: 'binary'
   originalContent?: string
@@ -15,12 +12,8 @@ export type MobileBinaryDiffResult = {
   mimeType?: string
 }
 
-// Builds a data URI for a binary image diff, preferring the modified (post-change)
-// bytes. Only falls back to the original for a genuine deletion — when the modified
-// side is absent (not binary). Returns null for non-previewable binaries and when
-// the modified side IS a binary image but arrives empty (e.g. a relay working-tree
-// read or a >size-cap file), since showing the pre-change image there would be
-// silently stale rather than the current content.
+// Falls back to the original bytes only for a genuine deletion; a size-capped/relay
+// modify returns null instead of the stale pre-change image.
 export function mobileDiffImageDataUri(result: MobileBinaryDiffResult): string | null {
   if (result.isImage !== true) {
     return null

mobile/src/files/mobile-file-tab-doc.ts

@@ -21,9 +21,8 @@ export type MobileFileTabDocRequest = {
   diffSource?: 'staged' | 'unstaged' | 'branch' | 'commit'
 }
 
-// Loads the previewable doc for a session file tab: git diffs (text or image),
-// previewable images, HTML source, or plain text. Throws 'binary_file',
-// 'file_too_large', or the RPC error message; callers map those to error docs.
+// Throws 'binary_file'/'file_too_large'/the RPC error message; callers map those
+// to error docs.
 export async function resolveMobileFileTabDoc(
   client: FileTabDocClient,
   request: MobileFileTabDocRequest