feat: support Qoder environment variables

[heimanba]

What changed

• add environment_variables to Agent declarations and Session creation options
• map variables to Qoder's three distinct wire formats:
  • Forward Template: top-level object
  • managed Session: KEY=VALUE;... string
  • Forward Session: config.environment_variables object
• reject Agent environment variables for non-Qoder providers
• document .env interpolation and Qoder runtime behavior
• add coverage for parsing, inheritance, validation, and both Session delivery modes

Why

Qoder supports runtime environment variables on Forward Templates and Sessions, but the provider adapter did not expose the capability. Users could keep values in .env, yet those values could not be propagated through agents.yaml into the Qoder runtime.

Impact

Qoder users can now declare an environment-variable map on an Agent and source its values from .env. The same declaration works for managed and Forward delivery while preserving each API's required request shape.

Validation

• bun run --cwd packages/sdk typecheck
• 74 focused SDK tests
• Biome checks on changed files
• repository pre-push verification harness
• real Qoder managed Session: remote Bash read the expected .env-sourced value
• real Qoder Forward Template/Session: remote Bash read the expected .env-sourced value

Temporary Qoder Sessions, Agents, Templates, Identities, and Environments used for the live test were deleted afterward.