model.context_length short-circuits get_model_context_length ahead of every
provider source, but nothing told the user the number they saw was their own
pin rather than provider metadata (#66168). Keep the pin semantics (custom
endpoints depend on it) and make it visible instead:
- agent/context_pin.py: is_context_pinned / context_pin_suffix for renderers,
and warn_once_on_pin_disagreement, called from _resolve_context_length at
agent init. The advertised value comes from LOCAL sources only (endpoint
cache, models.dev disk cache, hardcoded catalog) so the check never adds a
network probe or latency to startup; one warning per (model, pin) per process.
- "(pinned)" label on every CLI surface that renders the window: welcome
banner, /model switch summary, /usage current-context line, wide status bar.
- No new config key or env var; the numeric value used is unchanged.
Fixes#66168
