Rename API_KEY and API_ENDPOINT to HIRUNDO_API_KEY and HIRUNDO_API_ENDPOINT

[mishana]

Overview

Rename the environment variable names used throughout the SDK to be namespaced under the HIRUNDO_ prefix, avoiding potential conflicts with other tools.

Changes Required

• API_KEY → HIRUNDO_API_KEY
• API_HOST → HIRUNDO_API_HOST

Scope

• Update all references in source code
• Update any .env.example or similar template files
• Update README and any other docs referencing these env vars
• Consider adding a deprecation warning or backward-compatibility shim for the old names

Acceptance Criteria

• All code references updated from API_KEY to HIRUNDO_API_KEY
• All code references updated from API_HOST to HIRUNDO_API_HOST
• Documentation and README updated accordingly
• Tests updated to use the new variable names
• No remaining references to the old names (or a deprecation shim in place)