feat: add dynamic template variables to messages.responsePrefix (#923)

Adds support for template variables in `messages.responsePrefix` that
resolve dynamically at runtime with the actual model used (including
after fallback).

Supported variables (case-insensitive):
- {model} - short model name (e.g., "claude-opus-4-5", "gpt-4o")
- {modelFull} - full model identifier (e.g., "anthropic/claude-opus-4-5")
- {provider} - provider name (e.g., "anthropic", "openai")
- {thinkingLevel} or {think} - thinking level ("high", "low", "off")
- {identity.name} or {identityName} - agent identity name

Example: "[{model} | think:{thinkingLevel}]" → "[claude-opus-4-5 | think:high]"

Variables show the actual model used after fallback, not the intended
model. Unresolved variables remain as literal text.

Implementation:
- New module: src/auto-reply/reply/response-prefix-template.ts
- Template interpolation in normalize-reply.ts via context provider
- onModelSelected callback in agent-runner-execution.ts
- Updated all 6 provider message handlers (web, signal, discord,
  telegram, slack, imessage)
- 27 unit tests covering all variables and edge cases
- Documentation in docs/gateway/configuration.md and JSDoc

Fixes #923

src/auto-reply/reply/normalize-reply.ts

@@ -1,9 +1,15 @@
 import { stripHeartbeatToken } from "../heartbeat.js";
 import { HEARTBEAT_TOKEN, isSilentReplyText, SILENT_REPLY_TOKEN } from "../tokens.js";
 import type { ReplyPayload } from "../types.js";
+import {
+  resolveResponsePrefixTemplate,
+  type ResponsePrefixContext,
+} from "./response-prefix-template.js";
 
 export type NormalizeReplyOptions = {
   responsePrefix?: string;
+  /** Context for template variable interpolation in responsePrefix */
+  responsePrefixContext?: ResponsePrefixContext;
   onHeartbeatStrip?: () => void;
   stripHeartbeat?: boolean;
   silentToken?: string;
@@ -36,13 +42,18 @@ export function normalizeReplyPayload(
     text = stripped.text;
   }
 
+  // Resolve template variables in responsePrefix if context is provided
+  const effectivePrefix = opts.responsePrefixContext
+    ? resolveResponsePrefixTemplate(opts.responsePrefix, opts.responsePrefixContext)
+    : opts.responsePrefix;
+
   if (
-    opts.responsePrefix &&
+    effectivePrefix &&
     text &&
     text.trim() !== HEARTBEAT_TOKEN &&
-    !text.startsWith(opts.responsePrefix)
+    !text.startsWith(effectivePrefix)
   ) {
-    text = `${opts.responsePrefix} ${text}`;
+    text = `${effectivePrefix} ${text}`;
   }
 
   return { ...payload, text };