fix(plugin): remove duplicate mission briefing injection

[madtank]

Summary

Removes duplicate mission briefing injection in the ax-platform plugin that was inflating prompt context and causing overflow risk.

Changes

• In extension/channel/ax-channel.ts async dispatch path:
  • BodyForAgent now sends raw message
  • removed SystemContext: missionBriefing
  • removed now-unused local briefing variables
• In sync dispatch path:
  • BodyForAgent now sends raw message
  • removed SystemContext: missionBriefing
  • removed now-unused local briefing variables
  • updated dispatcher log to remove context-length reference

Intent

Keep mission briefing injection in a single place (before_agent_start hook via prependContext) and avoid double-context construction.

Validation

No build/typecheck scripts are defined in extension/package.json; attempted script checks report missing scripts.