chore: release v0.8.121

[miguel-heygen]

Release v0.8.121.

• Media Use: a credentials folder means no HeyGen credential, not a crash (fix(media-use): a credentials folder means no HeyGen credential, not a crash #4995).
• Media Use: skip a .env folder when loading HeyGen credentials (fix(media-use): skip a .env folder when loading HeyGen credentials #4988).
• Core: one pure owner for composition length and root size (refactor(core): one pure owner for composition length and root size #4990).

Version bumps and changelog only. Merging publishes: the workflow tags the merge commit, publishes npm and cuts the GitHub release.

No visible change

Release bookkeeping only; no UI change in this PR.

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Project	Status	Preview	Updated
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[github-actions]

Edit accuracy: accurate 1556 (base branch 1556), smooth 1444 of those

The gate passes.
Smoothness is reported in the artifact, not gated. A case fails only if it fails 2 of 3 runs.

Quarantined, measured but not gated (0)