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

docs/gateway/configuration.md

@@ -1184,6 +1184,31 @@ streaming, final replies) across channels unless already present.
 If `messages.responsePrefix` is unset, no prefix is applied by default.
 Set it to `"auto"` to derive `[{identity.name}]` for the routed agent (when set).
 
+#### Template variables
+
+The `responsePrefix` string can include template variables that resolve dynamically:
+
+| Variable | Description | Example |
+|----------|-------------|---------|
+| `{model}` | Short model name | `claude-opus-4-5`, `gpt-4o` |
+| `{modelFull}` | Full model identifier | `anthropic/claude-opus-4-5` |
+| `{provider}` | Provider name | `anthropic`, `openai` |
+| `{thinkingLevel}` | Current thinking level | `high`, `low`, `off` |
+| `{identity.name}` | Agent identity name | (same as `"auto"` mode) |
+
+Variables are case-insensitive (`{MODEL}` = `{model}`). `{think}` is an alias for `{thinkingLevel}`.
+Unresolved variables remain as literal text.
+
+```json5
+{
+  messages: {
+    responsePrefix: "[{model} | think:{thinkingLevel}]"
+  }
+}
+```
+
+Example output: `[claude-opus-4-5 | think:high] Here's my response...`
+
 WhatsApp inbound prefix is configured via `channels.whatsapp.messagePrefix` (deprecated:
 `messages.messagePrefix`). Default stays **unchanged**: `"[clawdbot]"` when
 `channels.whatsapp.allowFrom` is empty, otherwise `""` (no prefix). When using