docs(spec): add helper-instance slice to the agent-runtime spec

The v0 draft covered identity and message targeting for concurrent agents but not spawn mechanics or write-safety for the shared files the session-context split doesn't isolate. I added a section for the motivating case (a second Claude in one project doing lookups and safe task updates): ai --helper spawn with automatic AI_AGENT_ID, a tiered read/write contract where helpers make scoped single-heading org edits and file-wide passes plus all git mutation stay primary-only, light helper startup, and helper wrap-up. Phase 1.5 sequences the slice independently of the runtime-neutral phases 2-6.

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