fix(claude): the slash picker is detected on Claude 2.1.283 and follows colour-only selection changes

docs/plans/2026-09-27-claude-slash-picker-colour-select.md

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+# Claude slash picker: a colour-only selection change republishes the picker (#1253)
+
+## Problem
+`detectSlashPicker` reads the selected row from relative foreground colour. `HeadlessTerminal.scheduleFlush` gates emission on the `plain` + `recent` TEXT only, so arrowing through the picker, which changes only the highlight colour, emits no `screen` frame. `ClaudeCodeHeadless` never reparses the picker, and `claude.slash-picker` keeps the old selected row until the text changes. This predates #1236. Also, nothing pins the producer: replacing `slashPicker: this.pickerState` with an empty picker survived every picker suite.
+
+## Constraint (why the gate is text-only)
+Serialising cell attributes on every flush is the 60 Hz snapshot churn that once pinned main at ~80% CPU (agent-code#390; the gate's own comment). The fix must not make the common frame attribute-aware.
+
+## Decisions (defaults)
+- **Only while a picker is visible** (the last parsed state had one), the gate also compares a picker-selection signature: which picker row carries the selected colour, read from the live grid for the picker's rows only (a handful of cells). With no picker, the hot path is unchanged.
+- **Evidence first:** record a real Claude picker PTY sequence into `packages/claude-code-headless/testing/fixtures/`:
+  1. open with `/`;
+  2. filter;
+  3. arrow down twice (colour-only changes);
+  4. Esc.
+
+  Drive the `claude` CLI directly in a PTY, not the Agent Code app. No model call is made, but decode and redact the whole fixture before committing it.
+- **Tests:** a real-frame test on the recording asserts the `conditions` snapshots through open → filtered → selection moved (colour only) → closed. The producer is pinned, so an empty picker in the evaluator input fails.
+
+## Out of scope
+Attribute-aware gating for anything else. The composer styling-flip case stays covered by the prompt gate's bounded staleness.