docs: add WhisperGate architecture and implementation plan

docs/IMPLEMENTATION.md

@@ -0,0 +1,21 @@
+# WhisperGate Implementation Notes
+
+## Decision path
+
+WhisperGate evaluates in strict order:
+
+1. channel check (discord-only)
+2. bypass sender check
+3. message ending symbol check
+4. fallback to no-reply model override
+
+## Why before_model_resolve
+
+- deterministic
+- no LLM dependency
+- low overhead
+- uses built-in override path (`providerOverride` + `modelOverride`)
+
+## Known limitation
+
+This does not fully skip OpenClaw prompt assembly. It reduces provider-side LLM usage by routing no-reply turns to a deterministic API.