feat(routing): add DISABLE_CONTEXT_WINDOW_CHECKS bypass for the direct-request input/context check (#10927)

Rescoped from #10606 — see PR body for the full rationale (combo-routing half made moot by #10162's advisory-only architecture, chatCore.ts hard-reject bypass retains real value).

Validated in an isolated worktree boarded onto origin/release/v3.8.50:
- 65/65 focused unit tests pass (chatcore-model-output-cap-wiring + feature-flags-settings).
- check-file-size, check-changelog-integrity: OK.
- typecheck:core: clean.
- check-complexity / check-cognitive-complexity: OK, both under baseline.

Co-authored-by: JxnLexn <10897478+JxnLexn@users.noreply.github.com>

.env.example

@@ -417,6 +417,15 @@ ALLOW_API_KEY_REVEAL=false
 # by OMNIROUTE_CHAT_MAX_HEAVY_IN_FLIGHT and the heap-pressure shed instead. Set a positive
 # value only on memory-constrained deployments that need a hard ceiling.
 # OMNIROUTE_CHAT_HARD_MAX_MESSAGES=0
+
+# Skip OmniRoute's local context-window and max-input-token check for direct
+# single-model requests. Default: false (dangerous opt-in).
+# The upstream provider still enforces its real limits, so enabling this can
+# replace an early OmniRoute 400 with an upstream context-length error.
+# Prompt compression and the model's own output-token cap remain active.
+# Also configurable from Dashboard > Settings > Feature Flags; no restart is
+# required. Used by: src/shared/utils/featureFlags.ts and open-sse/handlers/chatCore.ts.
+# DISABLE_CONTEXT_WINDOW_CHECKS=false
 # How long a heavy request waits for heavyweight capacity before a retryable 503.
 # A short bounded wait serializes agent bursts instead of an instant 503; 0 = instant.
 # Default 2000 (2s).