fix(env): forward ANTHROPIC_DEFAULT_*_MODEL to Agent SDK sub-call

[MukundaKatta]

Why

Closes #1258.

The Agent SDK's top-level call inherits ANTHROPIC_DEFAULT_SONNET_MODEL (and OPUS/HAIKU) from process.env, but spawned sub-call subprocesses (Task tool, sub-agents) drop them and fall back to the SDK's hardcoded default. With a non-Anthropic preset configured via ANTHROPIC_DEFAULT_SONNET_MODEL, sub-calls leak literal claude-sonnet-4-6 to the proxy and silently bill the user for Sonnet calls they did not request - see the issue's smoking-gun OpenRouter generation log.

The action.yml already forwards these env vars to the bun process, and parse-sdk-options.ts already spreads process.env into sdkOptions.env. The top-level call honours the override, so that path works. Sub-calls go through a fresh CLI subprocess that reads ~/.claude/settings.json for env, not the parent's process env, so the override is dropped.

What

Mirror the three model-alias env vars into settings.env from setupClaudeCodeSettings. The CLI propagates settings.env to every session it spawns, including sub-calls, via the documented settings layer (Settings.env in the Agent SDK types).

• User-provided settings.env entries always win on conflict; the action only fills in keys the user did not set.
• Empty/unset env vars are skipped - no settings.env is written when none of the three are present.
• All three (ANTHROPIC_DEFAULT_SONNET_MODEL, ANTHROPIC_DEFAULT_OPUS_MODEL, ANTHROPIC_DEFAULT_HAIKU_MODEL) are handled in the same loop.

Tested

Added five unit tests under base-action/test/setup-claude-code-settings.test.ts in a new model-alias env var forwarding (issue #1258) describe block:

• All three env vars mirrored into settings.env.
• Only the env vars that are set get forwarded.
• No settings.env is created when none are set.
• User-provided settings.env keys take precedence over env-var values.
• Empty env var values are skipped.

beforeEach/afterEach snapshot and restore the three env vars so the suite is hermetic.

• bun test base-action/test/setup-claude-code-settings.test.ts
• Manual: set ANTHROPIC_DEFAULT_SONNET_MODEL=@preset/... in workflow env, run the action, observe sub-call requests now hit the preset rather than literal claude-sonnet-4-6 (per the reproduction in Action ignores ANTHROPIC_DEFAULT_SONNET_MODEL for internal Agent SDK sub-calls (regression in v1.0.104 / claude-cli 2.1.118) #1258).