Slice 10 of issue #440 — full composer + dock + model picker rewrite. 38 commits squashed.

## Highlights

- **Composer joined-card shell**: new DockCard + DockSegment primitives in @opencode-ai/ui, dock widgets share the L34 joined-card layout. Hybrid composer shell renders permission UI inline.
- **prompt-input.tsx 1602 → ~570 lines**: split into prompt-input/{store-types, editor-serialize, editor-imperatives, comment-routing, history-navigation, popover-controllers, editor-input, keydown, model-controls}. No behavior change — all factories injected via deps; popover↔editor-input cycle resolved via mutable forward-ref.
- **DockWidgetHeader unified**: single 36px collapsed header with 30px chev IconButton (3+3 breathing) — consolidates Followup, Revert, Todo widgets.
- **DOCK_MOTION contract**: composer/dock/widgets share `{ visualDuration: 0.3, bounce: 0 }` via packages/app/src/pages/session/composer/motion.ts. Aligns with global useSpring convention.
- **Model picker rewrite**: variant control merged into model trigger; thinking-level promoted from separate popover to inline section. `[data-picker-content]` guard prevents nested popover dismiss.
- **Send button + ContextUsage parity**: 30px outer / 16px inner glyph, theme-locked colors, larger stop glyph.
- **Followup defaults to queue mode**: drops the legacy steer override.
- **Control heights**: single 30px dense tier across composer; sidebar trailing actions step down to 26 per nesting rule.
- **Picker contracts**: tighter padding, narrower width (340), dropped sticky labels.

## Post-review fixes (CI + 2 review rounds)

- Critical: DockCard manual classList merge (single key with spaces) was throwing DOMException InvalidCharacterError — replaced with Solid native class+classList co-rendering.
- CI typecheck: PopoverControllers types aligned with upstream solid-list `Accessor<string | null>` (was `undefined`).
- CI e2e: dropped stale `prompt-variant-control` selector (variant folded into model picker).
- Send button e2e contract: 32 → 30px to match dense tier.
- Editor serialize: fixed double-newline regression for blank rows.
- Keydown: removed dead `if (stopping())` branches (both arms identical).
- Custom slash command id includes source to prevent workspace+user collision.
- Model trigger label: pinned to `min-w-[80px] max-w-[180px]` to balance short names ("GPT-5.5") vs long ("DeepSeek V4 Pro") without jiggling the workspace control.

## Systemic finding (tracked under #34)

PawWork sets `html { font: var(--type-body) }` → root font-size 13px. This means **Tailwind rem-based units render at 81% of their pixel-named values**: `h-9` = 29.25px, not 36px. Multiple visual bugs in this slice (chev clipping, header height) traced back to this. Remediation: use absolute pixels (`h-[36px]`) for grid-pinned heights; avoid Tailwind's pixel-named scale for 4pt-grid alignment. DESIGN.md docs debt to be filed under #34.

## Hand-test backlog (open after merge)

- #32 dark theme regression sweep
- #33 single-prompt E2E

## Files of note

- packages/ui/src/components/dock-card.tsx (new)
- packages/app/src/pages/session/composer/{motion.ts, use-dock-collapse.ts, dock-widget-header.tsx, session-composer-region.tsx, session-todo-dock.tsx}
- packages/app/src/components/prompt-input/* (9 new files)
- packages/app/e2e/composer/composer-slice-10.spec.ts (new)

Closes part of #440.