Fix Computer Use evidence feed migration

[mimeding]

Summary

• update the Computer Use evidence-pack test to construct the current SubagentFeed type
• preserve the original tool-call id and goal text while sourcing the Computer Use kind id from the subagent capability registry

Why

Current main migrated Computer Use progress reporting onto the shared subagent feed, but one evidence-pack test still used the removed ComputerUseFeed initializer. That breaks the test target before the Computer Use PR stack can prove its own changes.

Validation

• git diff --check
• OSAURUS_DISABLE_KEYCHAIN_FOR_TESTS=1 OSAURUS_TEST_ROOT=/tmp/osaurus-test-computer-use-feed swift test --package-path Packages/OsaurusCore --scratch-path /Users/mmeding/Developer/Claude/Projects/osaurus/Packages/OsaurusCore/.build --filter ComputerUseEvidencePackTests --skip-update — 7 tests passed
• Independent review: one non-blocking registry drift suggestion was addressed; Grok review was unavailable/inconclusive and did not block implementation