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/agents/pi-embedded-runner/run.ts

@@ -263,6 +263,8 @@ export async function runEmbeddedPiAgent(
         sessionId: params.sessionId,
         workspaceDir: resolvedWorkspace,
         messageProvider: params.messageProvider ?? undefined,
+        trigger: params.trigger,
+        channelId: params.messageChannel ?? params.messageProvider ?? undefined,
       };
       if (hookRunner?.hasHooks("before_model_resolve")) {
         try {
@@ -715,6 +717,7 @@ export async function runEmbeddedPiAgent(
           const attempt = await runEmbeddedAttempt({
             sessionId: params.sessionId,
             sessionKey: params.sessionKey,
+            trigger: params.trigger,
             messageChannel: params.messageChannel,
             messageProvider: params.messageProvider,
             agentAccountId: params.agentAccountId,