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/config/types.messages.ts

@@ -44,8 +44,21 @@ export type MessagesConfig = {
   messagePrefix?: string;
   /**
    * Prefix auto-added to all outbound replies.
-   * - string: explicit prefix
+   *
+   * - string: explicit prefix (may include template variables)
    * - special value: `"auto"` derives `[{agents.list[].identity.name}]` for the routed agent (when set)
+   *
+   * Supported template 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}` - current thinking level (`high`, `low`, `off`)
+   * - `{identity.name}` or `{identityName}` - agent identity name
+   *
+   * Example: `"[{model} | think:{thinkingLevel}]"` → `"[claude-opus-4-5 | think:high]"`
+   *
+   * Unresolved variables remain as literal text (e.g., `{model}` if context unavailable).
+   *
    * Default: none
    */
   responsePrefix?: string;