docs(tui): rename "Ink TUI" to just "TUI" throughout user-facing surfaces

"Ink" is the React reconciler — implementation detail, not branding.
Consistent naming: the classic CLI is the CLI, the new one is the TUI.

Updated docs: user-guide/tui.md, user-guide/cli.md cross-link, quickstart,
cli-commands reference, environment-variables reference.

Updated code: main.py --tui help text, server.py user-visible setup
error, AGENTS.md "TUI Architecture" section.

Kept "Ink" only where it is literally the library (hermes-ink internal
source comments, AGENTS.md tree note flagging ui-tui/ as a React/Ink dir).

tui_gateway/server.py

@@ -1985,7 +1985,7 @@ def _cli_exec_blocked(argv: list[str]) -> str | None:
         return "bare `hermes` is interactive — use `/hermes chat -q …` or run `hermes` in another terminal"
     a0 = argv[0].lower()
     if a0 == "setup":
-        return "`hermes setup` needs a full terminal — run it outside the Ink UI"
+        return "`hermes setup` needs a full terminal — run it outside the TUI"
     if a0 == "gateway":
         return "`hermes gateway` is long-running — run it in another terminal"
     if a0 == "sessions" and len(argv) > 1 and argv[1].lower() == "browse":