RFC: Add optional `conversation.tool` override

[jonathansantilli]

A single trace record can include conversations produced by different tools. Top-level tool alone cannot represent mixed-tool records accurately.

Problem

Developers often use multiple AI tools within the same editing session. For example, a tab-autocomplete engine for quick completions alongside a chat-based agent for larger refactors. If both contributions appear in one trace record, the top-level tool can only represent one of them. The other tool's provenance is either lost or pushed into unstructured metadata, which reduces the value of a standard attribution format.

Proposal

1. Add optional conversation.tool with the same structure as top-level tool.
2. Define precedence:
   • If conversation.tool is present, it applies to that conversation.
   • Otherwise, use top-level tool as the default.

Why This Should Be Added

1. Preserves precise provenance in mixed-tool workflows.
2. Avoids forcing a common attribution dimension into vendor-specific metadata.
3. Keeps existing top-level behavior unchanged when absent.

Compatibility

Backward-compatible optional addition.

Scope

This is a data-model expressiveness change for attribution only. It does not propose protocol or UI behavior.