fix(core): use resolved absolute path for file references in agent mode

[christso]

Summary

• formatSegment in agent mode was using segment.path (relative display path) instead of segment.resolvedPath (absolute) when rendering <file: path="..."> tags
• This caused agents running in isolated workspaces (e.g. copilot CLI with isolation: per_test) to fail when trying to read input_files, since the relative paths did not exist in the workspace directory
• Now uses segment.resolvedPath with a fallback to display path for synthetic/test segments

Root cause

In segment-formatter.ts, the agent mode branch read segment.path (the human-readable display path set by message-processor.ts) rather than segment.resolvedPath (the absolute path also set by message-processor.ts during file resolution). When the agent runs in an isolated workspace with a different cwd, the relative path doesn't resolve to any file.

Reproduction

# eval.yaml with isolation: per_test workspace
input_files:
  - snippets/data.csv
input: "Summarize this CSV."

The agent receives <file: path="snippets/data.csv"> instead of <file: path="/abs/path/to/evals/test/snippets/data.csv">, tries to read it relative to its workspace, and fails with ERROR: missing-file.

Changes

File	Change
packages/core/src/evaluation/formatting/segment-formatter.ts	Use resolvedPath (absolute) in agent mode, fallback to path
packages/core/test/evaluation/formatting/segment-formatter.test.ts	New test file covering formatSegment, formatFileContents, and hasVisibleContent

Refactoring note

The formatSegment function operates on JsonObject (untyped), making it easy to reference the wrong property (path vs resolvedPath). A typed file segment interface would prevent this class of bug at compile time. Kept this PR focused on the fix; happy to follow up with a typing improvement if desired.

Test plan

• New unit tests for formatSegment agent mode (resolvedPath, fallback, missing path)
• Existing input-files-shorthand.test.ts passes (59 tests)
• Full packages/core/ test suite passes (1626 tests)
• Typecheck passes
• Lint passes

🤖 Generated with Claude Code

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