Remove context status-line meter (#17420)

Addresses #17313

Problem: The visual context meter in the status line was confusing and
continued to draw negative feedback, and context reporting should remain
an explicit opt-in rather than part of the default footer.

Solution: Remove the visual meter, restore opt-in context remaining/used
percentage items that explicitly say "Context", keep existing
context-usage configs working as a hidden alias, and update the setup
text and snapshots.

codex-rs/core/src/config/mod.rs

@@ -324,8 +324,7 @@ pub struct Config {
 
     /// Ordered list of status line item identifiers for the TUI.
     ///
-    /// When unset, the TUI defaults to: `model-with-reasoning`, `context-remaining`, and
-    /// `current-dir`.
+    /// When unset, the TUI defaults to: `model-with-reasoning` and `current-dir`.
     pub tui_status_line: Option<Vec<String>>,
 
     /// Ordered list of terminal title item identifiers for the TUI.