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/response-prefix-template.ts

@@ -0,0 +1,97 @@
+/**
+ * Template interpolation for response prefix.
+ *
+ * Supports variables like `{model}`, `{provider}`, `{thinkingLevel}`, etc.
+ * Variables are case-insensitive and unresolved ones remain as literal text.
+ */
+
+export type ResponsePrefixContext = {
+  /** Short model name (e.g., "gpt-5.2", "claude-opus-4-5") */
+  model?: string;
+  /** Full model ID including provider (e.g., "openai-codex/gpt-5.2") */
+  modelFull?: string;
+  /** Provider name (e.g., "openai-codex", "anthropic") */
+  provider?: string;
+  /** Current thinking level (e.g., "high", "low", "off") */
+  thinkingLevel?: string;
+  /** Agent identity name */
+  identityName?: string;
+};
+
+// Regex pattern for template variables: {variableName} or {variable.name}
+const TEMPLATE_VAR_PATTERN = /\{([a-zA-Z][a-zA-Z0-9.]*)\}/g;
+
+/**
+ * Interpolate template variables in a response prefix string.
+ *
+ * @param template - The template string with `{variable}` placeholders
+ * @param context - Context object with values for interpolation
+ * @returns The interpolated string, or undefined if template is undefined
+ *
+ * @example
+ * resolveResponsePrefixTemplate("[{model} | think:{thinkingLevel}]", {
+ *   model: "gpt-5.2",
+ *   thinkingLevel: "high"
+ * })
+ * // Returns: "[gpt-5.2 | think:high]"
+ */
+export function resolveResponsePrefixTemplate(
+  template: string | undefined,
+  context: ResponsePrefixContext,
+): string | undefined {
+  if (!template) return undefined;
+
+  return template.replace(TEMPLATE_VAR_PATTERN, (match, varName: string) => {
+    const normalizedVar = varName.toLowerCase();
+
+    switch (normalizedVar) {
+      case "model":
+        return context.model ?? match;
+      case "modelfull":
+        return context.modelFull ?? match;
+      case "provider":
+        return context.provider ?? match;
+      case "thinkinglevel":
+      case "think":
+        return context.thinkingLevel ?? match;
+      case "identity.name":
+      case "identityname":
+        return context.identityName ?? match;
+      default:
+        // Leave unrecognized variables as-is
+        return match;
+    }
+  });
+}
+
+/**
+ * Extract short model name from a full model string.
+ *
+ * Strips:
+ * - Provider prefix (e.g., "openai/" from "openai/gpt-5.2")
+ * - Date suffixes (e.g., "-20251101" from "claude-opus-4-5-20251101")
+ * - Common version suffixes (e.g., "-latest")
+ *
+ * @example
+ * extractShortModelName("openai-codex/gpt-5.2") // "gpt-5.2"
+ * extractShortModelName("claude-opus-4-5-20251101") // "claude-opus-4-5"
+ * extractShortModelName("gpt-5.2-latest") // "gpt-5.2"
+ */
+export function extractShortModelName(fullModel: string): string {
+  // Strip provider prefix
+  const slash = fullModel.lastIndexOf("/");
+  const modelPart = slash >= 0 ? fullModel.slice(slash + 1) : fullModel;
+
+  // Strip date suffixes (YYYYMMDD format)
+  return modelPart.replace(/-\d{8}$/, "").replace(/-latest$/, "");
+}
+
+/**
+ * Check if a template string contains any template variables.
+ */
+export function hasTemplateVariables(template: string | undefined): boolean {
+  if (!template) return false;
+  // Reset lastIndex since we're using a global regex
+  TEMPLATE_VAR_PATTERN.lastIndex = 0;
+  return TEMPLATE_VAR_PATTERN.test(template);
+}