[Feature] Introduce unified AgentRuntime and SourceCollector contracts

[jerry609]

Goal

Define stable contracts so all agent flows share one lifecycle and one integration surface.

Scope

• Add src/paperbot/core/abstractions/agent_runtime.py
• Add src/paperbot/application/ports/source_collector.py
• Define shared schemas: AgentMessage, AgentResult, AgentError
• Keep compatibility adapters for existing agents

Acceptance

• Contract types compile and are imported by at least one route/workflow
• Current agents can be adapted without behavior regression
• Contract includes run_id / trace_id / stage / agent_name

[jerry609]

Completed in commit 432a088 on branch feat/harvest-aligned-base-v2.