aibridge: delete deprecated estimate_tokens wrapper — fully migrated

cdc24d8 migrated all 5 call sites to shared::model_matrix::ModelMatrix.
Grep across the workspace confirms zero remaining callers (only doc
comments in the new module reference the old name). Wrapper was there
to smooth the transition; transition is done.

Leaves a 3-line breadcrumb comment pointing to the new location so
anyone opening this file sees the migration history. The deprecated
wrapper itself is 4 lines deleted.

Workspace warnings still at 0 (both lib + tests).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/aibridge/src/context.rs

@@ -13,12 +13,9 @@
 use std::collections::HashMap;
 use std::sync::OnceLock;
 
-/// Rough token count. `chars / 4` ceiling. See module docs for why
-/// this heuristic is sufficient.
-#[deprecated(note = "Use shared::model_matrix::ModelMatrix::estimate_tokens instead")]
-pub fn estimate_tokens(text: &str) -> usize {
-    (text.chars().count() + 3) / 4
-}
+// `estimate_tokens` moved to `shared::model_matrix::ModelMatrix::estimate_tokens`
+// (cdc24d8). All callers migrated; the deprecated wrapper that stood in its
+// place has been removed since it had zero external consumers.
 
 /// Phase 21 — per-model context windows, mirroring the TS table in
 /// `tests/multi-agent/agent.ts`. Anchored on each model's documented