feat(labels): render Linear labels as Org tags on issue headings

An issue's Linear labels now appear as Org tags on its heading (** TODO [#B] Title :bug:backend:), so the org-native gestures work: filter by tag, build a tag agenda, sparse-tree on :bug:. Before this, labels lived only in the :LINEAR-LABELS: drawer, which Org's tag machinery can't see.

pearl--label-name-to-tag slugifies a label name to a tag: downcase, non-[[:alnum:]_] runs become a single underscore, Unicode letters preserved. pearl--label-tags builds the deduplicated set, first occurrence winning on a collision. The renderer appends them, and pearl-edit-labels rewrites them through pearl--set-heading-label-tags after updating the drawer.

This is render-only. The :LINEAR-LABELS: drawer stays the source of truth, hand-edited heading tags are ignored by save and the dirty scan and get rewritten from Linear on the next change or fetch, and the way to change labels is still pearl-edit-labels. Bidirectional tag editing (heading tags back to Linear) is out of scope.

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