Stamp gloop version + commit SHA into runs.jsonl and observe.jsonl entries

Problem

During the fishtank Big Fish experiment (2026-07-26), the driver's local gloop checkout was stale at 26a94a6 — one commit behind the merge of the observability feature (#47 / PR #48). The first three sessions of the experiment (design + two workers) therefore ran with no observe.jsonl instrumentation, and nothing in any artifact revealed this: runs.jsonl entries look identical regardless of which gloop built them. The gap was only caught by manually grepping the checkout for observe.jsonl after noticing the file was missing.

Proposal

Stamp the gloop version into every ledger entry:

• Add a gloop field to each runs.jsonl and observe.jsonl record: the package version plus the git SHA of the gloop checkout when available (e.g. "gloop":"0.1.0+092223f").
• Print the same string in the startup banner.

This makes "which gloop produced this run" answerable from the ledger alone — the exact provenance question the experiment tripped over.

Acceptance

• Every new runs.jsonl / observe.jsonl line carries the version+SHA field.
• Version resolution degrades gracefully outside a git checkout (falls back to package version).
• Test covering ledger-entry shape.

Locate the code paths that write runs.jsonl and observe.jsonl, then inspect the startup-banner entry point and existing version handling. Add coverage for the ledger-entry shape and verify that every new line includes version and SHA when available, while falling back to the package version outside a git checkout.