Give injected agent submit Enter a longer settle delay

The trailing CR that submits orchestrator-injected input was written
only 15ms after the body, inside TUI agents' paste-coalescing window,
so codex (and other paste-detecting agents) intermittently swallowed it
as a newline and left the message composed but unsent. Centralize the
per-piece timing in a pure pieceWriteDelay helper: keep 15ms between
body lines but give the final lone Enter a 100ms settle gap so the
agent closes the preceding burst and registers the CR as submit. Covers
send_input, send_message, timers, and the spawn initial prompt (all go
through writeInput).

Resolves the codex composer-submit TODO item.

CHANGELOG.md

@@ -16,6 +16,9 @@ loosely follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
   over MCP.
 
 ### Fixed
+- Injected agent input now sends the submit Enter as a separated,
+  settled keystroke so messages reliably submit instead of sometimes
+  sitting unsent in the composer.
 - Codex agents are no longer reported idle while a turn is still
   running.
 - Slow MCP tool calls such as `wait_for_pattern` no longer block later