Add hc_agent contracts module (RunnerEnvelope, RunSummary, StructuredError)

[binrogithub]

Motivation

• Centralize canonical runner payload contracts and a structured error helper to avoid duplication across the codebase and to guarantee stable JSON field names for external consumers.
• Provide typed definitions for envelopes and summaries to improve IDE/type-checking ergonomics and make payload shapes explicit.

Description

• Added a new hc_agent package with hc_agent/core/contracts.py that defines StructuredError, build_structured_error, RunSummary, and RunnerEnvelope as TypedDict types and helper function with literal JSON field names preserved (type, message, details, status, started_at, finished_at, duration_ms, metadata, run_id, summary, error).
• Exported the canonical contracts from hc_agent/core/__init__.py (exports: RunSummary, RunnerEnvelope, StructuredError, build_structured_error).
• Added top-level hc_agent/__init__.py to mark the new package.
• This PR only introduces the canonical contracts module; other modules were not modified in this change set and will need to be updated in follow-up changes to import from hc_agent.core and remove local duplicates.

Testing

• No automated tests were executed for these changes.

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Codex Task