chore: split the label manifest into baseline and per-repo overlays

[jan-kubica]

Follow-up to #66 and #67. labels.yml mixed org-wide process labels with ones only meaningful inside the application repo, so opting folio or a Rust package into the sync would have created an "outlook plugin" label in a DOCX library. That is why #66 defaulted the target to stella rather than all.

The split

labels.yml          # baseline: 26 labels every repo that opts in receives
labels/stella.yml   # overlay: 16 labels specific to stella/stella

The script reads the baseline, then merges labels/<repo>.yml on top when it exists. An overlay entry wins on a name collision, so a repo can retune a shared label without forking the manifest.

Moved to the overlay: 🤖 AI, 🎨 ux, 🔌 external data, 📬 outlook plugin, 🧮 table. Promoted into the baseline: run-pr-lint and invalid, which are governance and a GitHub default rather than product surface.

Declaring what was already there

labels/stella.yml also declares eleven labels stella/stella has carried all along that the manifest had never heard of: the six react-doctor: * CI labels, 📝 docx-editor, 🌍 i18n, 🐞 bug, agent-feedback, and javascript.

They previously survived a prune only because prune skips labels still in use. Now they survive because they are declared. That is the difference between apply-with-prune being safe and being meaningful: anything undeclared is now undeclared on purpose.

plan can preview deletions

Prune previously ran only under apply-with-prune, which deletes immediately, so there was no way to review a destructive run before it happened. plan now reports prune candidates and writes nothing; only apply-with-prune deletes.

Verification

Local plan against both repo shapes.

stella/stella, which has an overlay and was synced this morning:

=== stella/stella
create=0 update=0 prune=0 keep=0 mode=plan

Zero on every counter is the load-bearing result: baseline plus overlay reproduces the live state exactly, so the split introduced no drift. prune=0 confirms the overlay accounts for every label actually present.

stella/folio, which has no overlay:

=== stella/folio
  ~ update  bug (description "Something isn't working" -> "Something is not working")
  + create  chore
  + create  docs
  ~ update  enhancement (color #a2eeef -> #0cd187, ...)
  + create  needs approval
  + create  perf
  + create  run-ci
  + create  run-pr-lint
  + create  size/L ... size/XXL
  + create  ⚡ epic
  + create  💭 triage
  + create  📦 dependencies
  + create  🚧 blocked
  + create  🛠️  dev
  + create  🧹 refactor
  - prune   documentation
create=18 update=2 prune=1 keep=0 mode=plan

Baseline only. No 📬 outlook plugin, no 🧮 table, no react-doctor: *. That is the whole point of the split.

shellcheck clean.

After merge

The stella/stella caller pins the reusable workflow to a SHA, and this PR changes that workflow (the sparse checkout needs labels/). The pin must be bumped in a follow-up or the overlay will not be checked out and the sync will silently fall back to baseline only.

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