Fix Claude auth normalization and Studio flags

[armand0e]

Follow-up to #18 for the two unresolved P2 review findings that could not be pushed to the contributor fork:

• normalize configured Claude OAuth placeholders before enabling subscription auth or validating api.base_url
• share Claude effort/fallback flag construction with Studio native terminal sessions
• add regression coverage for both paths

Validation:

• pytest -q tests/test_config.py tests/test_runner.py tests/test_studio.py — 275 passed
• full suite on the combined Add Claude Code subscription auth and passthroughs #18 + fixes tree with a fixed wide console — 570 passed, 25 skipped
• ruff check on changed files — passed
• wheel build — passed

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