feat(hooks): add trigger and channelId to plugin hook agent context (#28623)

* feat(hooks): add trigger and channelId to plugin hook agent context

Adds `trigger` and `channelId` fields to `PluginHookAgentContext` so
plugins can determine what initiated the agent run and which channel
it originated from, without session-key parsing or Redis bridging.

trigger values: "user", "heartbeat", "cron", "memory"
channelId values: "telegram", "discord", "whatsapp", etc.

Both fields are threaded through run.ts and attempt.ts hookCtx so all
hook phases receive them (before_model_resolve, before_prompt_build,
before_agent_start, llm_input, llm_output, agent_end).

channelId falls back from messageChannel to messageProvider when the
former is not set. followup-runner passes originatingChannel so queued
followup runs also carry channel context.

* docs(changelog): note hook context parity fix for #28623

---------

Co-authored-by: Vincent Koc <vincentkoc@ieee.org>

src/commands/agent.ts

@@ -279,6 +279,7 @@ function runAgentAttempt(params: {
     sessionId: params.sessionId,
     sessionKey: params.sessionKey,
     agentId: params.sessionAgentId,
+    trigger: "user",
     messageChannel: params.messageChannel,
     agentAccountId: params.runContext.accountId,
     messageTo: params.opts.replyTo ?? params.opts.to,