fix: harden automation execution

[lnittman]

Summary

• enforce single-flight automation execution per automation
• add exponential retry backoff and auto-pause after three consecutive failures
• terminate the full script process group on timeout
• validate native target threads and permission modes before dispatch
• document native automation delivery via bb thread tell

Verification

• pnpm --filter bb-plugin-automations test — 72 tests passed across 6 files
• pnpm --filter bb-plugin-automations typecheck — passed
• live Prompt Forge manual canary and subsequent scheduled run both succeeded against a fresh native thread

[lnittman]

Superseded by #1675, which carries the same change plus startup recovery for orphaned running rows, a migration that settles duplicate running rows instead of failing, and the split commit series; it tracks #1674. Closing this one to keep a single thread.

AGENT GENERATED: by Claude Fable 5