terminal: tail pill above floating chrome + autoscroll fix (#375)

Two bugs on the agent terminal page after the #362 overhaul:

## 1. `↓ N new` pill clipped by the composer

The fixed-overlay composer (z-index 30, agent.css) sits in the
root stacking context. The pill — `position: absolute` inside
`.live.terminal` with no z-index — defaults to its document-order
position in the body's stacking order, which the composer covers.

Fix: bump `.agent-main .tail-pill { z-index: 35 }` so the pill
participates in the root stacking context above the composer.
Scoped to the agent-page overlay layout — the shared `.tail-pill`
rule stays untouched (the dashboard's in-page layout doesn't need
the bump).

## 2. Autoscroll-on-new-message not firing when the operator was
   already at the bottom

`afterAppend()` in terminal.js was calling `isNearBottom()` AFTER
appending the new row. The new row's own height is already in
`scrollHeight` at that point, so for any row taller than the
NEAR_BOTTOM_PX threshold (48px — easily passed by a multi-line
message body, a tool-result summary, a markdown block), the check
returns false and the pill shows + scroll stays put.

Fix: capture `wasNearBottom = isNearBottom()` BEFORE the
`log.appendChild(...)` in each of `row` / `details` /
`detailsDiff`, pass it into `afterAppend(wasNearBottom)`. Now the
auto-scroll triggers whenever the operator was visually at the
bottom an instant before the row landed, regardless of the new
row's height.

Same shared `@hive/shared/terminal.js` is used by the dashboard
+ per-agent UI + the upcoming /flow.html page, so both pages
inherit the fix.

## Validation

`npm run build` clean.

Bundle deltas: shared terminal bundle re-inlined into both consumers
unchanged in size (the wasNearBottom variable is a single bool, no
measurable delta). Agent CSS +0.1kb (z-index property).

Browser smoke test isn't possible from inside iris's container —
worth eyeballing post-deploy:
  - With the operator scrolled to bottom, a tall message lands and
    the view scrolls to keep it visible (instead of pinning the
    pill).
  - The pill appears above the composer when the operator is
    scrolled up and new messages land.

Closes #375.

frontend/packages/agent/src/agent.css

@@ -548,9 +548,12 @@ pre.diff {
   overflow: auto;
 }
 /* Tail pill (↓ N new): nudged up so it floats clear of the composer
-   rather than colliding with the frosted bar. */
+   rather than colliding with the frosted bar. z-index bumped above
+   the composer (z-30) so the pill sits on the top layer instead of
+   being clipped by the floating chrome (issue #375). */
 .agent-main .tail-pill {
   bottom: calc(var(--agent-composer-h) + 0.6em);
+  z-index: 35;
 }
 
 /* Composer chrome — used to live inside `.terminal-wrap`; now lives