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/types.ts

@@ -5,6 +5,13 @@ export type BlockReplyContext = {
   timeoutMs?: number;
 };
 
+/** Context passed to onModelSelected callback with actual model used. */
+export type ModelSelectedContext = {
+  provider: string;
+  model: string;
+  thinkLevel: string | undefined;
+};
+
 export type GetReplyOptions = {
   onReplyStart?: () => Promise<void> | void;
   onTypingController?: (typing: TypingController) => void;
@@ -13,6 +20,9 @@ export type GetReplyOptions = {
   onReasoningStream?: (payload: ReplyPayload) => Promise<void> | void;
   onBlockReply?: (payload: ReplyPayload, context?: BlockReplyContext) => Promise<void> | void;
   onToolResult?: (payload: ReplyPayload) => Promise<void> | void;
+  /** Called when the actual model is selected (including after fallback).
+   * Use this to get model/provider/thinkLevel for responsePrefix template interpolation. */
+  onModelSelected?: (ctx: ModelSelectedContext) => void;
   disableBlockStreaming?: boolean;
   /** Timeout for block reply delivery (ms). */
   blockReplyTimeoutMs?: number;