feat(agent-interface): add durable plan contract

[vutuanlinh2k2]

Summary

• add a closed, typed durable-plan contract for Claude Code, Codex, and OpenCode
• add the plan submission host with explicit submit, confirm, and withdraw phases
• add typed plan continuations and required execution outcomes so callers distinguish completion from waiting for a decision
• add the plan.submitted stream record and schema coverage
• publish the contract as a minor @tangle-network/agent-interface release

Related issue

• Supports tangle-network/agent-dev-container#3417

Verification

• pnpm check-types
• pnpm test (all workspace packages; agent-interface 96 tests, agent-core 418 tests)
• pnpm build
• git diff --check