shared terminal + docs: migrate sticky-scroll + backfill prose (#714 batch 1)

Substantial prose migration from @hive/shared/terminal.js (the
shared HiveTerminal factory backing #msgflow + #live across both
dashboard and per-agent UIs) into a new docs subsection.

Added to docs/web-ui.md as a new ### Shared terminal pane
subsection under ## Shape (shared by both) — ~78 lines of new
substantive prose:

- **api shape**: row / details / detailsDiff factory contract
- **Sticky-bottom + snap animation**: stickToBottom semantics +
  140ms ease-out vs 500ms browser default + 24px short-circuit;
  per-frame target re-eval extends destination through
  mid-animation mutations
- **Mid-animation scroll-event guard**: smoothScrollingUntil
  timestamp swallows the rAF-driven scroll events so the eased
  positions don't flip stickToBottom false partway
- **Post-append MutationObserver**: catches renderer mutations
  after api.row returns (badges, multi-line bodies, tool panes)
  + why programmatic scrollTop writes don't feedback-loop
- **Backfill + SSE**: history/stream envelope shape (seq, events),
  kind-aware seq dedupe at the boundary, .no-anim during replay,
  optional streamFactory for SharedWorker integration
- **linkify**: text-node-only autolink, XSS-safe by construction,
  trailing-punctuation strip

Collapsed in terminal.js (cookies en passant):
- #400 (snap animation timing — closed) × 3 → docs pointers
- #393 (post-append MutationObserver — closed) × 1 → docs pointer
- #375 (pre-append nearBottom snapshot — closed) × 1 → docs pointer
- #448 (streamFactory SharedWorker hook — closed) × 1 → docs pointer
- #163 (seq dedupe + onStreamOpen resync — closed) × 2 → drop
  cookies; substance lives in docs
- #233 (linkify) × 1 → docs pointer + terminal.css cookie scrub

terminal.js: 8 → 0 #NNN cookies (100% reduction).
terminal.css: 1 → 0 issue-ref cookies (remaining 1 match is a
hex color literal).
Net effect: ~50 lines of substantive WHY-prose moved out of
shared frontend into docs/web-ui.md, where it documents the
factory's contract for both consumer pages.

frontend/packages/shared/src/terminal.css

@@ -213,7 +213,7 @@ details.row > pre.diff-body .diff-ctx { color: var(--fg); }
   border-radius: 0;
 }
 .live .row .md a { color: var(--cyan); text-decoration: underline; }
-/* Auto-linkified bare URLs in plain rows + tool-body blocks (issue #233). */
+/* Auto-linkified bare URLs in plain rows + tool-body blocks. */
 .live .row a { color: var(--cyan); text-decoration: underline; }
 .live .row a:hover { color: var(--fg); }
 .live .row .md strong { color: inherit; font-weight: bold; }