feat: attach project name to trace metadata

[johnbuccola]

Adds a short project field derived from the existing cwd metadata. This lets Langfuse traces be grouped by checkout while retaining the full cwd and Git branch context. Verification: 183 tests pass.

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[atoomic]

@johnbuccola — this is solving the same need I have (group traces by what the run was for),
and I've just opened #61 approaching it from the tags side, so one concrete datum in case it's useful here.

On a self-hosted Langfuse v3.212.0, GET /api/public/traces filters first-class on
tags, userId, sessionId, name, environment, release and version — but not on
metadata. Verified directly against the API: ?tags=project:foo returns the matching
traces, while there's no equivalent ?metadata= parameter.

So the two look complementary rather than overlapping: metadata is right for context you read
once you've opened a trace (and project derived from cwd is genuinely nicer to read than a
full path), while tags are what you can actually slice a cost or latency report by. Worth
having both — I don't think this PR and mine are in tension.