Add endpoint resolution context and HTTP signer/executor for hc_agent

[binrogithub]

Motivation

• Provide a unified endpoint resolution mechanism that merges runtime environment and config values and supports HC_AGENT_ and HC_ENDPOINT_ overrides.
• Support table-driven service templates with sensible fallbacks and special casing for services like rds and aom, and expose the resolved endpoint to HTTP executor and signer helpers.

Description

• Add hc_agent/core/endpoints.py which implements EndpointContext, merges config and environment (HC_AGENT_REGION, HC_AGENT_BASE, HC_AGENT_BASE_DOMAIN) and loads endpoint overrides from config (endpoints, endpoint_overrides, agent.endpoints) and environment variables prefixed with HC_ENDPOINT_.
• Implement template-driven resolution with DEFAULT_TEMPLATES and SERVICE_TEMPLATES to map services to patterns including rds.{region}.{base} and aom.{region}.{base} with fallbacks to region-less templates.
• Add hc_agent/core/http_executor.py and hc_agent/core/signer.py which consume the EndpointContext and resolve service hosts for HTTP requests and signing operations respectively.
• No skills from the AGENTS.md catalog were used for this change.

Testing

• No automated tests were run for this change.

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