Fix test-finder workflow: Enable developer extension and add token metrics

[tlongwell-block]

Problem

The daily test-finder workflow was missing the developer extension, which is required for Goose to:

• Edit source files to add tests
• Run cargo test to verify tests pass
• Apply fixes when tests fail

Without this extension, Goose cannot fulfill its core purpose in this workflow.

Solution

1. Enable developer extension: Added --with-builtin developer flag to the goose run command
2. Add usage metrics: Extract and display token usage in PR descriptions for cost visibility

Changes

• ✅ Minimal change to enable developer extension (1 flag addition)
• 📊 Added token metrics extraction step
• 📈 Display metrics in PR body (total, input, output tokens)

Technical Details

• The session file is stored at a known path within the container's session directory
• Metrics are extracted from the JSONL metadata using jq (already installed)
• Gracefully handles missing session files without failing the workflow

Benefits

• Fixes core functionality: Goose can now actually create and test code
• Cost transparency: Each PR shows exactly how many tokens were used
• Usage tracking: Helps optimize prompts and monitor AI resource consumption
• No breaking changes: Fully backward compatible

Testing

• Verified --with-builtin flag syntax locally
• Confirmed session path must be within goose session directory
• Checked that jq is already installed in the workflow

[tlongwell-block]

Looking at this PR now to assess if it will work correctly...