'git-ai stats' now resolves foreign prompts if attributions are present but the prompt is missing.

[mm-zacharydavison]

I'm not 100% sure how this occurred in our repo, but I've got cases whereby we have attestations in a git-ai note for a commit, but no prompts, which causes git-ai stats to give the wrong stats:

❯ git notes --ref=ai show f7dea7ffa83838d774e4deaac0bb653e16ff5f8a                                                                                11:02:23
apps/server/src/mcp/mcp-jwt.guard.spec.ts
  10d7219 24,83,90
apps/server/src/mcp/mcp-jwt.guard.ts
  10d7219 62
---
{
  "schema_version": "authorship/3.0.0",
  "git_ai_version": "1.0.21",
  "base_commit_sha": "f7dea7ffa83838d774e4deaac0bb653e16ff5f8a",
  "prompts": {}
}

❯ git-ai stats f7dea7ffa83838d774e4deaac0bb653e16ff5f8a                                                                                           11:02:30
you  ████████████████████████████████████████ ai
     100%                                   0%

This PR fixes this bug by trying to resolve foreign prompts (inspired by how virtual_attestations.rs does it).

I'm not entirely sure if this shouldn't be root-cause fixed elsewhere, i'm not sure how this could happen (but most likely it's squash flow, this came from a Github PR that was squashed).

However, since the performance impact only hits if this situation happens, and only in the git-ai stats workflow, I think it's a safe precaution to add.

after fix:

apps/server/src/mcp/mcp-jwt.guard.spec.ts
  10d7219 24,83,90
apps/server/src/mcp/mcp-jwt.guard.ts
  10d7219 62
---
{
  "schema_version": "authorship/3.0.0",
  "git_ai_version": "1.0.21",
  "base_commit_sha": "f7dea7ffa83838d774e4deaac0bb653e16ff5f8a",
  "prompts": {}
}

❯ git-ai stats f7dea7ffa8                                                                                                                  12:14:15
[git-ai] Stats command found commit: f7dea7ffa83838d774e4deaac0bb653e16ff5f8a refname: f7dea7ffa8
[git-ai] Resolving 1 foreign prompts for stats calculation
[git-ai] Found foreign prompt 10d7219 in note path d3/5e2f2e862b8d7b4730263ee384936ee97b20a6
you  ██████████████████████████████████░░░░░░ ai
     86%                                  14%
     100% AI code accepted | waited 3m for ai