`conceal.lua` hides the `[[id|` prefix and `]]` suffix with conceal
extmarks (refreshed on edit), leaving the label as a styled `HephLink`;
`conceallevel=2` + empty `concealcursor` reveal the raw `[[id|Name]]` on
the cursor's line so it stays editable. The `[[` picker now inserts the
labelled `[[id|Name]]` form (readable + conceal-ready; collapses to bare
on save). e2e asserts the conceal extmarks + conceallevel.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
