ci: Publish a nightly code-ownership map

[ellemouton]

Adds a nightly workflow that measures who has been working on each package and
file in this repo — commits per author over a 12-month window — and commits the
result to .github/code-owners.json.

Why

It feeds issue triage. lumos has an assignment workflow (lightninglabs/lumos#766)
that routes a new issue to whoever owns the code it touches, and issues get filed
there that track work here — so it needs ownership data for this repo.

It fetches this published file rather than cloning and recomputing. A repo is
the only place that can authoritatively answer who works on its code, and doing
it elsewhere needed a cross-repo token to do something this repo can do for
itself. Hence: each repo publishes its own, nobody computes anyone else's.

The map is worth having independently too — its diff shows ownership moving
between people over time, and a wrong answer can be argued with in a PR rather
than rediscovered by hand.

Measurement choices worth reviewing

• Packages are credited once per commit, not once per file touched, so one
  sweeping rename doesn't outweigh months of focused work.
• Both per-package and per-file counts are kept, because they disagree often.
  Measured on the sibling repo, 28% of tracked files have a different top author
  than the package containing them — and the margin differs even where the winner
  doesn't.
• Authors resolve through an explicit email→handle table, not name matching,
  which doesn't work on this history: one contributor's display name resembles
  their handle not at all, and two others each commit under more than one name.
  This table needs updating when someone joins or changes address.
• Merge commits are excluded (they name whoever pressed the button), as are
  generated code and vendored trees.

Operational notes

• Runs 03:42 UTC, an hour ahead of the consumer's 04:42 run, so the map it
  fetches is same-day rather than a day stale.
• Opens a PR — it never pushes to a protected branch, and refuses at runtime
  if its branch is ever configured as one.
• Most nights produce no diff and therefore no PR; ownership drifts slowly.
• Needs BOT_GITHUB_TOKEN to exist in this repo, same as the other bot
  workflows. If it isn't set, the job fails at checkout.

🤖 Generated with Claude Code