Add easing and duration tokens, set transition defaults

Every transition in the project ran on `ease` or `ease-out` — the browser
default, and the least characterful curve available. `ease` eases both ways: it
spends its opening accelerating before it settles, which blunts the start of a
change that ought to feel like a reply. One curve now does nearly all of it,
leaving immediately and settling slowly, so a change looks like a response to
whatever caused it.

Durations are named for their job rather than numbered. The agent's change
marks keep their 600ms as `linger`, which is the one duration here that is not
feedback — a fade nobody is waiting on.

Setting Tailwind's transition defaults to the same tokens means the fifteen
`transition-colors` utilities already written pick the curve up without being
edited, and a utility and a hand-written rule beside it now resolve to the same
120ms and the same cubic-bezier.

`--ease-*: initial` first, so `ease-in` stops resolving to Tailwind's curve.
One curve is the point; a second one nobody chose is not.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>