fix(oss-commit-sync): stop reporting our own exports as lost trailers

[sydorovdmytro]

Summary

A green sync-from-oss run on vcluster-pro printed:

::notice::Anchor healed from content: 887d668e -> c6abe8ca (4 OSS commit(s) already
present in staging/github.com/loft-sh/vcluster but not recorded by a readable
Oss-Commit trailer).

All 4 commits carried a Monorepo-Commit trailer: our own exports. Their content is in staging/ because that is where it was authored, and they will never carry an Oss-Commit trailer, because that trailer records an import. content_anchor did not distinguish them from external commits, so the notice fired after every export with a count that grew until the next external import recorded a fresh trailer, and health reported the same range as stale-anchor.

A genuine trailer loss (squash-merged sync PR) produces the identical message, so the annotation that has to stay trustworthy was the one crying wolf.

• classify_healed_range splits the healed range by provenance: Monorepo-Commit present means our export (expected), neither trailer means an import whose record was lost.
• Import annotates only the unrecorded case and points at the merge method; healing over our own exports is a plain log line.
• Health judges stale-anchor on the unrecorded count alone and reports both classes in the step summary.
• New outputs: healed-export-count, healed-unrecorded-count, redundant-export-count, redundant-unrecorded-count. healed-count / redundant-count keep their totals.

Test plan

• make test-oss-commit-sync — 64 tests pass, including three new cases: export-only healing (no ::notice::), a mixed range (1 export + 1 unrecorded, notice fires), and health treating an export-only lag as stale-anchor=false. The existing lost-trailer case now also asserts the unrecorded classification.
• make check-docs clean, make lint (actionlint + zizmor) clean, shellcheck adds no new findings.

Follow-up

Merging does not ship this: oss-commit-sync/v1 must be advanced to the merged commit, otherwise vcluster-pro's sync-from-oss and sync-to-oss keep running the old code.

Closes DEVOPS-1238