Add external turn actors to execution principals

[chubes4]

Summary

• add an immutable optional external turn actor to execution principals
• propagate actor identity through canonical chat, conversation context, and run metadata
• keep actor identity outside capabilities, ownership, approvals, credentials, and run-control authority
• document the trusted channel integration contract and omitted-actor compatibility

Tests

• composer test
• composer phpstan
• focused principal, run-control, and authorization smoke suites

All passed.

Closes #520.

AI assistance

OpenAI GPT-5.6 Sol via OpenCode general coding subagents implemented the principal contract, propagation, documentation, and regression coverage; OpenCode reviewed the resulting commit and verification evidence under Chris Huber's direction. Chris remains responsible for every line.