fix(workspace): allow git base_commit SHAs

[christso]

Closes #1236

What changed

• treat full workspace.repos[].checkout.base_commit SHAs as local checkout targets after clone for source.type: git repos
• keep remote ls-remote resolution for branch/tag-style refs
• add a regression test that reproduces the failure against a local bare git remote

Verification

• bun test ./test/evaluation/workspace/repo-manager.test.ts in packages/core
• bun run build
• bun run lint
• bun run test

Red

bun apps/cli/src/cli.ts eval issue-1236-valid-repro.eval.yaml --dry-run --target default --targets issue-1236-targets.yaml on origin/main failed during workspace setup with:

Failed to materialize repos: Ref '6e446b722627e9df017b22e391fa63320362d8c7' not found on remote https://github.com/EntityProcess/agentv\n```\n\n### Green\nThe same command on this branch completed workspace setup and ran the test case instead of erroring; the run ended as a normal assertion failure from the mock target:\n\n```text\n1/1   ⚠️ noop | default → default-dry-run | 0% FAIL\n```\n\n### Note\nIssue claiming on the project board was blocked in this environment because the configured `gh` token is missing the `read:project` scope required by `gh project item-list`.\n

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