Add ingest HTTP export with bearer token support

[negillett]

Summary

• Add ingest_request_headers() and post_execution_event() helpers.
• Send Authorization: Bearer when INTENTPROOF_INGEST_TOKEN is set.

Test plan

• python -m pytest -q

Review

• No blocking issues found.

[cursor]

PR Summary

Medium Risk
Adds outbound HTTP POST behavior and token-based authorization header handling, which can impact integrations and error handling on network failures.

Overview
Adds a new intentproof.exporter module with helpers to build ingest request headers (optionally adding Authorization: Bearer ... from INTENTPROOF_INGEST_TOKEN) and to POST JSON execution events to a hosted ingest URL with basic status/error handling.

Exports these helpers from intentproof.__init__ and adds tests covering header behavior with and without the env token.

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