fix(tui): address code review findings

Medium fixes:
- textInput.tsx: prevent silent data loss when async paste resolves
  after user types — fall back to raw text insert at current cursor
  instead of dropping the content entirely
- useComposerState.ts: tighten looksLikeDroppedPath to require a
  second '/' or '.' for bare absolute paths, avoiding unnecessary
  RPC round-trips for pasted text like /api or /help
- useComposerState.ts: add cross-reference comment linking to the
  canonical _detect_file_drop() in cli.py
- osc52.ts: add 500ms timeout via Promise.race so terminals that
  do not support OSC52 clipboard queries cannot hang paste

Low fixes:
- terminalSetup.ts: export isRemoteShellSession and reuse in
  terminalParity.ts and useComposerState.ts (was inlined 3 times)
- useComposerState.ts: extract insertAtCursor helper, replacing 3
  copies of the lead/tail spacing logic
- useComposerState.ts: remove redundant gw from handleTextPaste
  useCallback dependency array
- terminalSetup.test.ts: add EACCES (read-only keybindings.json)
  and unterminated block comment test coverage

ui-tui/src/lib/osc52.ts

@@ -48,18 +48,21 @@ export function parseOsc52ClipboardData(data: string): null | string {
   }
 }
 
-export async function readOsc52Clipboard(querier: null | OscQuerier): Promise<null | string> {
+export async function readOsc52Clipboard(querier: null | OscQuerier, timeoutMs = 500): Promise<null | string> {
   if (!querier) {
     return null
   }
 
-  const response = await querier.send<OscResponse>({
+  const timeout = new Promise<undefined>(resolve => setTimeout(resolve, timeoutMs))
+  const query = querier.send<OscResponse>({
     request: buildOsc52ClipboardQuery(),
     match: (r: unknown): r is OscResponse => {
       return !!r && typeof r === 'object' && (r as OscResponse).type === 'osc' && (r as OscResponse).code === 52
     }
   })
 
+  const response = await Promise.race([query, timeout])
+
   await querier.flush()
 
   return response ? parseOsc52ClipboardData(response.data) : null