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>

docs/gateway/configuration.md

@@ -1517,6 +1517,25 @@ See [Messages](/concepts/messages) for queueing, sessions, and streaming context
 `responsePrefix` is applied to **all outbound replies** (tool summaries, block
 streaming, final replies) across channels unless already present.
 
+Overrides can be configured per channel and per account:
+
+- `channels.<channel>.responsePrefix`
+- `channels.<channel>.accounts.<id>.responsePrefix`
+
+Resolution order (most specific wins):
+
+1. `channels.<channel>.accounts.<id>.responsePrefix`
+2. `channels.<channel>.responsePrefix`
+3. `messages.responsePrefix`
+
+Semantics:
+
+- `undefined` falls through to the next level.
+- `""` explicitly disables the prefix and stops the cascade.
+- `"auto"` derives `[{identity.name}]` for the routed agent.
+
+Overrides apply to all channels, including extensions, and to every outbound reply kind.
+
 If `messages.responsePrefix` is unset, no prefix is applied by default. WhatsApp self-chat
 replies are the exception: they default to `[{identity.name}]` when set, otherwise
 `[openclaw]`, so same-phone conversations stay legible.