feat: per-channel responsePrefix override (#9001)

* feat: per-channel responsePrefix override

Add responsePrefix field to all channel config types and Zod schemas,
enabling per-channel and per-account outbound response prefix overrides.

Resolution cascade (most specific wins):
  L1: channels.<ch>.accounts.<id>.responsePrefix
  L2: channels.<ch>.responsePrefix
  L3: (reserved for channels.defaults)
  L4: messages.responsePrefix (existing global)

Semantics:
  - undefined -> inherit from parent level
  - empty string -> explicitly no prefix (stops cascade)
  - "auto" -> derive [identity.name] from routed agent

Changes:
  - Core logic: resolveResponsePrefix() in identity.ts accepts
    optional channel/accountId and walks the cascade
  - resolveEffectiveMessagesConfig() passes channel context through
  - Types: responsePrefix added to WhatsApp, Telegram, Discord, Slack,
    Signal, iMessage, Google Chat, MS Teams, Feishu, BlueBubbles configs
  - Zod schemas: responsePrefix added for config validation
  - All channel handlers wired: telegram, discord, slack, signal,
    imessage, line, heartbeat runner, route-reply, native commands
  - 23 new tests covering backward compat, channel/account levels,
    full cascade, auto keyword, empty string stops, unknown fallthrough

Fully backward compatible - no existing config is affected.
Fixes #8857

* fix: address CI lint + review feedback

- Replace Record<string, any> with proper typed helpers (no-explicit-any)
- Add curly braces to single-line if returns (eslint curly)
- Fix JSDoc: 'Per-channel' → 'channel/account' on shared config types
- Extract getChannelConfig() helper for type-safe dynamic key access

* fix: finish responsePrefix overrides (#9001) (thanks @mudrii)

* fix: normalize prefix wiring and types (#9001) (thanks @mudrii)

---------

Co-authored-by: Gustavo Madeira Santana <gumadeiras@gmail.com>

src/telegram/bot-message-dispatch.test.ts

@@ -1,3 +1,4 @@
+import type { Bot } from "grammy";
 import { beforeEach, describe, expect, it, vi } from "vitest";
 
 const createTelegramDraftStream = vi.hoisted(() => vi.fn());
@@ -72,16 +73,25 @@ describe("dispatchTelegramMessage draft streaming", () => {
       removeAckAfterReply: false,
     };
 
+    const bot = { api: { sendMessageDraft: vi.fn() } } as unknown as Bot;
+    const runtime = {
+      log: vi.fn(),
+      error: vi.fn(),
+      exit: () => {
+        throw new Error("exit");
+      },
+    };
+
     await dispatchTelegramMessage({
       context,
-      bot: { api: {} },
+      bot,
       cfg: {},
-      runtime: {},
+      runtime,
       replyToMode: "first",
       streamMode: "partial",
       textLimit: 4096,
       telegramCfg: {},
-      opts: {},
+      opts: { token: "token" },
       resolveBotTopicsEnabled,
     });