chore(plans): add input-history and model-selection plans

plans/input-history.md:
- Implementation plan for shell-style up/down arrow message history
  in SimpleInput, deriving history from session log conversation data
- Covers prop threading, history derivation, navigation state,
  keybinding details, modal parity, and test cases

plans/model-selection.md:
- Three-phase plan for model visibility and control: display current
  model, model picker at spawn, mid-session model switching via Zellij

plans/PLAN-tool-result-display.md:
- Updates to tool result display plan (pre-existing changes)

plans/subagent-visibility.md:
- Updates to subagent visibility plan (pre-existing changes)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

plans/input-history.md

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+# Input History (Up/Down Arrow)
+
+## Summary
+
+Add shell-style up/down arrow navigation through past messages in SimpleInput. History is derived from the conversation data already parsed from session logs -- no new state management, no server changes.
+
+## How It Works Today
+
+1. Server parses JSONL session logs, extracts user messages with `role: "user"` (`conversation.py:57-66`)
+2. App.js stores parsed conversations in `conversations` state, refreshed via SSE on `conversation_mtime_ns` change
+3. SessionCard receives `conversation` as a prop but does **not** pass it to SimpleInput
+4. SimpleInput has no awareness of past messages
+
+## Step 1: Pipe Conversation to SimpleInput
+
+Pass the conversation array from SessionCard into SimpleInput so it can derive history.
+
+- `SessionCard.js:165-169` -- add `conversation` prop to SimpleInput
+- Same for the QuestionBlock path if freeform input is used there (line 162) -- skip for now, QuestionBlock is option-based
+
+**Files**: `dashboard/components/SessionCard.js`
+
+## Step 2: Derive User Message History
+
+Inside SimpleInput, filter conversation to user messages only.
+
+```js
+const userHistory = useMemo(
+  () => (conversation || []).filter(m => m.role === 'user').map(m => m.content),
+  [conversation]
+);
+```
+
+This updates automatically whenever the session log changes (SSE triggers conversation refresh, new prop flows down).
+
+**Files**: `dashboard/components/SimpleInput.js`
+
+## Step 3: History Navigation State
+
+Add refs for tracking position in history and preserving the draft.
+
+```js
+const historyIndexRef = useRef(-1);  // -1 = not browsing
+const draftRef = useRef('');         // saves in-progress text before browsing
+```
+
+Use refs (not state) because index changes don't need re-renders -- only `setText` triggers the visual update.
+
+**Files**: `dashboard/components/SimpleInput.js`
+
+## Step 4: ArrowUp/ArrowDown Keybinding
+
+In the `onKeyDown` handler (after the autocomplete block, before Enter-to-submit), add history navigation:
+
+- **ArrowUp**: only when autocomplete is closed AND cursor is at position 0 (prevents hijacking multiline cursor movement). On first press, save current text to `draftRef`. Walk backward through `userHistory`. Call `setText()` with the history entry.
+- **ArrowDown**: walk forward through history. If past the newest entry, restore `draftRef` and reset index to -1.
+- **Reset on submit**: set `historyIndexRef.current = -1` in `handleSubmit` after successful send.
+- **Reset on manual edit**: in `onInput`, reset `historyIndexRef.current = -1` so typing after browsing exits history mode.
+
+### Cursor position check
+
+```js
+const atStart = e.target.selectionStart === 0 && e.target.selectionEnd === 0;
+```
+
+Only intercept ArrowUp when `atStart` is true. This lets multiline text cursor movement work normally. ArrowDown can use similar logic (check if cursor is at end of text) or always navigate history when `historyIndexRef.current !== -1` (already browsing).
+
+**Files**: `dashboard/components/SimpleInput.js`
+
+## Step 5: Modal Parity
+
+The Modal (`Modal.js:71`) also renders SimpleInput with `onRespond`. Verify it passes `conversation` through. The same SessionCard is used in enlarged mode, so this should work automatically if Step 1 is done correctly.
+
+**Files**: `dashboard/components/Modal.js` (verify, likely no change needed)
+
+## Non-Goals
+
+- No localStorage persistence -- history comes from session logs which survive across page reloads
+- No server changes -- conversation parsing already extracts what we need
+- No new API endpoints
+- No changes to QuestionBlock (option-based, not free-text history)
+
+## Test Cases
+
+| Scenario | Expected |
+|----------|----------|
+| Press up with empty input | Fills with most recent user message |
+| Press up multiple times | Walks backward through user messages |
+| Press down after browsing up | Walks forward; past newest restores draft |
+| Press up with text in input | Saves text as draft, shows history |
+| Press down past end | Restores saved draft |
+| Type after browsing | Exits history mode (index resets) |
+| Submit after browsing | Sends displayed text, resets index |
+| Up arrow in multiline text (cursor not at pos 0) | Normal cursor movement, no history |
+| New message arrives via SSE | userHistory updates, no index disruption |
+| Session with no prior messages | Up arrow does nothing |