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/terminalParity.ts

@@ -1,4 +1,4 @@
-import { detectVSCodeLikeTerminal, shouldPromptForTerminalSetup, type FileOps } from './terminalSetup.js'
+import { detectVSCodeLikeTerminal, isRemoteShellSession, shouldPromptForTerminalSetup, type FileOps } from './terminalSetup.js'
 
 export type MacTerminalHint = {
   key: string
@@ -18,7 +18,7 @@ export function detectMacTerminalContext(env: NodeJS.ProcessEnv = process.env):
 
   return {
     isAppleTerminal: termProgram === 'Apple_Terminal' || !!env['TERM_SESSION_ID'],
-    isRemote: !!(env['SSH_CONNECTION'] || env['SSH_TTY'] || env['SSH_CLIENT']),
+    isRemote: isRemoteShellSession(env),
     isTmux: !!env['TMUX'],
     vscodeLike: detectVSCodeLikeTerminal(env)
   }