RFC: Add optional `vcs.repository_url` for revision disambiguation

[jonathansantilli]

vcs.revision alone is often ambiguous outside a local checkout. In CI systems, centralized trace stores, and cross-repo analysis, the same revision value is not enough to identify source unambiguously.

Problem

A revision identifier without repository context is hard to resolve in distributed environments.

Proposal

Add optional vcs.repository_url:

"vcs": {
  "type": "git",
  "revision": "a1b2c3d...",
  "repository_url": "https://github.com/org/repo"
}

Why This Should Be Added

1. When traces are consumed outside the repository checkout (for example, in centralized attribution dashboards, CI pipelines, or cross-repo analysis), a revision hash alone is ambiguous. Two unrelated repositories can share the same short SHA prefix, and even full SHAs require the consumer to already know which repository to query. Including the repository URL removes that lookup ambiguity.
2. Makes trace records self-describing so they can be interpreted without additional context about where they came from.
3. Keeps the field optional for producers that only need local attribution.

Compatibility

Backward-compatible optional addition. Records without vcs.repository_url remain fully valid.

Scope

This is attribution record disambiguation only. It does not define storage, discovery, or integration protocol.