Add DeepAgents adapter

[mnvsk97]

Summary

• Add DeepAgents as a built-in harnessctl adapter with stdin/non-interactive execution, model/resume mapping, session discovery, and native session DB handoff pointers.
• Wire DeepAgents through run, shell, compare, doctor, context sync, and default config flows.
• Document README/adapter/session-file usage, custom OpenAI-compatible gateway configuration, changelog notes, launch post drafts, and DeepAgents smoke tests.

Validation

• bun test
• bun run typecheck
• bun run bundle
• bash test/sim-deepagents.sh
• bash test/real-deepagents-smoke.sh
• git diff --check
• Focused docs follow-up: bun test src/adapters/deepagents.test.ts src/adapters/registry.test.ts and git diff --check
• Manual local smoke: harnessctl run with DeepAgents as default returned harnessctl default deepagents ok using TrueFoundry-backed DeepAgents config

Risks / Rollout

• DeepAgents must be installed separately for the adapter to run.
• Provider-backed runs need DeepAgents credentials via /auth, env files, shell env, or harnessctl agent env; the live smoke falls back to a temporary local class_path model when no external credential is available.