Add Computer Use web form proof lane

[mimeding]

Summary

• Adds deterministic Computer Use web-form proof coverage for boring-form workflows.
• Hardens the live Computer Use path so ambiguous same-label targets stop before typing.
• Masks secure field values from model context, evidence, and value-change markers.
• Routes resolved text input through element actions instead of coordinate fallback, and proves declined dangerous app launch does not call driver open.

Validation

• git diff --check
• OSAURUS_DISABLE_KEYCHAIN_FOR_TESTS=1 OSAURUS_TEST_ROOT=/tmp/osaurus-pr1746-core swift test --package-path Packages/OsaurusCore --filter 'ComputerUseEvidencePackTests|AgentViewTests|ScreenContextDistillerTests|MockMacDriverContractTests'
• swift test --package-path Packages/OsaurusEvals --filter 'ComputerUseWebFormProofTests|ComputerUseLoopEvalTests'
• RUN_EVALS=0 STRICT=1 scripts/evals/computer-use-evidence.sh
• Final narrow rerun: OSAURUS_DISABLE_KEYCHAIN_FOR_TESTS=1 OSAURUS_TEST_ROOT=/tmp/osaurus-pr1746-agentview-final swift test --package-path Packages/OsaurusCore --filter AgentViewTests

Risk

• This changes Computer Use safety behavior in bounded cases only: ambiguous targets, secure-field rendering, resolved text entry, and declined app-open guardrails.
• Model-dependent live Computer Use evals are still opt-in; this PR includes deterministic local proof and artifact privacy validation.