fix(tui): harden terminal dimming and multiplexer copy (#14906)

- disable ANSI dim on VTE terminals by default so dark-background reasoning and accents stay readable
- suppress local multiplexer OSC52 echo while preserving remote passthrough and add regression coverage

ui-tui/packages/hermes-ink/src/ink/termio/osc.test.ts

@@ -0,0 +1,26 @@
+import { describe, expect, it } from 'vitest'
+
+import { shouldEmitClipboardSequence } from './osc.js'
+
+describe('shouldEmitClipboardSequence', () => {
+  it('suppresses local multiplexer clipboard OSC by default', () => {
+    expect(shouldEmitClipboardSequence({ TMUX: '/tmp/tmux-1/default,1,0' } as NodeJS.ProcessEnv)).toBe(false)
+    expect(shouldEmitClipboardSequence({ STY: '1234.pts-0.host' } as NodeJS.ProcessEnv)).toBe(false)
+  })
+
+  it('keeps OSC enabled for remote or plain local terminals', () => {
+    expect(shouldEmitClipboardSequence({ SSH_CONNECTION: '1', TMUX: '/tmp/tmux-1/default,1,0' } as NodeJS.ProcessEnv)).toBe(
+      true
+    )
+    expect(shouldEmitClipboardSequence({ TERM: 'xterm-256color' } as NodeJS.ProcessEnv)).toBe(true)
+  })
+
+  it('honors explicit env override', () => {
+    expect(shouldEmitClipboardSequence({ HERMES_TUI_CLIPBOARD_OSC52: '1', TMUX: '/tmp/tmux-1/default,1,0' } as NodeJS.ProcessEnv)).toBe(
+      true
+    )
+    expect(shouldEmitClipboardSequence({ HERMES_TUI_COPY_OSC52: '0', TERM: 'xterm-256color' } as NodeJS.ProcessEnv)).toBe(
+      false
+    )
+  })
+})