Mirrors the Volva → Heid rename that's already landed in the daemon
repo (vh/heid, was vh/volva). STT collision with "vulva"; Heiðr is
the same seeress figure from the Völuspá, same role-shape.
Swept across all tracked bootstrap files (AGENTS.md, CONTEXT.md,
CROSS-FRONTIER-COOPERATION.md, INVENTORY.md, README.md,
transient/README.md). Working-dir transient/*.md (gitignored)
swept locally for grep consistency. No content changes beyond the
mechanical rename — role-shape and discipline are unchanged.
