docs(sync-agent): design spec for dynamic per-integration sync engines

apps/kernel/src/agent/system-prompt.ts

@@ -74,7 +74,7 @@ You have five surfaces. Pick the one that matches the work, not the one that's m
 
 **computer_bash and the sandbox suite** for work that needs real Unix: pip, apt, ffmpeg, pandoc, git clones, multi-step builds, dev servers. Backed by an R2-mounted FUSE so \`/r2/*\` survives across turns. Slower than exec_code, much more capable. Use it when JS-only would be a stretch, not by default.
 
-**read_file / write_file / edit_file / list_files / find_files / move_file / delete_file / get_signed_url** for direct R2 ops on text content. \`edit_file\` is the surgical one: read-then-replace a unique anchor, no full-file rewrite. Use it for memory updates and any single-section edit. If your work around the FS call is more than three lines of logic, switch to \`env.FS\` inside exec_code instead.
+**read_file / grep_file / write_file / edit_file / list_files / find_files / move_file / delete_file / get_signed_url** for direct R2 ops on text content. \`grep_file\` runs regex server-side and returns only matching lines + context — reach for it whenever the file might exceed 64KB or you need to find by predicate across many files. \`read_file\` returns a line-number gutter; if you get \`truncated:true\` on a big file, switch to \`grep_file\` or call \`read_file\` again with \`{offset, limit}\` for a deeper slice. \`edit_file\` is the surgical one: read-then-replace a unique anchor, no full-file rewrite. Use it for memory updates and any single-section edit. If your work around the FS call is more than three lines of logic, switch to \`env.FS\` inside exec_code instead.
 
 **process_attachment** to understand binary content (PDFs, images) without bloating your context with raw bytes. PDFs go through Sonnet 4.6 server-side rendering, full text plus vision. For modifying binary files (writing PDFs, generating derivatives) use exec_code with a real parser.
 

apps/kernel/src/fs/env-fs.ts

@@ -196,6 +196,8 @@ const EXT_CONTENT_TYPE: Record<string, string> = {
   csv: "text/csv; charset=utf-8",
   tsv: "text/tab-separated-values; charset=utf-8",
   json: "application/json",
+  ndjson: "application/x-ndjson",
+  jsonl: "application/x-ndjson",
   yaml: "application/yaml",
   yml: "application/yaml",
   xml: "application/xml",