#32861 fix(home): keep chief avatars still by default

Chief profile pictures bounce by default after #30534. Default home.avatarMotion to Still, so avatars stay still both at idle and while working. Explicit Debug Settings overrides still apply.

No bouncing profile pictures and a very small, isolated PR. Two lines change in HomeTunables.swift: the default and its comment.

Observed / Not run

• Passed: Swift syntax, localization-defaults, feature-flags, git diff --check, and python3 scripts/cmux-next/prepush-gate.py.
• python3 scripts/verify-local.py --swift-changed also reports an existing workspace package grouping failure: missing CmuxDeviceHub. The workspace file and checker match base 6616d1764999.
• Independent read-only review: SHIP, no correctness findings. Confirmed defaults are resolved from code; only explicit overrides are persisted.
• Not run: native build, app tests, or live visual verification. No new tests for this default-only change; the existing Still path already prevents idle and working loops.
• Localization audit: no user-facing strings changed.

Changelog

Changed: Chief profile pictures stay still by default.

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Summary by cubic

Chief profile pictures no longer bounce by default. The home.avatarMotion tunable now defaults to .still instead of .float; motion remains available as an opt-in through Debug Settings.