fix(cli): defer Enter during rapid text input to prevent multiline paste splitting

[Ralojeil1]

Problem

When pasting multiline text into the Hermes CLI prompt without bracketed paste support (Windows Terminal, tmux, some terminals), newlines arrive as individual Enter key events. Each line gets submitted as a separate steering message instead of one complete message.

This is a major UX issue — pasting a 20-line context block floods the agent with 20 fragmented steering messages.

Root Cause

Without bracketed paste, pasted newlines are indistinguishable from manual Enter presses. The @kb.add('enter') handler fires for each newline, routing each line to the steering queue independently.

Fix: 50ms Timing Guard

Three changes in cli.py (9 lines total):

1. Tracker variable — _last_text_change = [None]
2. Timestamp update — _last_text_change[0] = time.monotonic() in _on_text_changed
3. Defer guard in handle_enter before normal input routing:

   if _last_text_change[0] and (time.monotonic() - _last_text_change[0]) < 0.05:
       return

The 50ms threshold cleanly separates:

Scenario	Time between chars
Paste (no bracketed paste)	< 1ms
Fast human typing	~80-100ms
Normal typing	> 50ms

Tested

• ✅ Windows Terminal (no bracketed paste) — multiline pastes buffer correctly
• ✅ Large pastes (731+ lines) accumulate fully, only submit on physical Enter
• ✅ No impact on normal or fast manual typing
• ✅ Functions across tmux, Windows Terminal, and any terminal without bracketed paste

Notes

Adapted from @iRonin's approach in PR #10997. This version updates the fix for the current codebase which has newer _on_text_changed logic (bracketed-paste leak stripping) not present in the original PR's base.

Closes #10994