#26503 Picker: keep Add harness in the harness strip

The picker keeps Add harness… in the existing harness strip. The compact and full pickers now end that strip with a clearly labelled + item; More models contains models only, and the model list no longer has a separate Add harness row.

Leo, word for word:

how the fuck was that your fucking first instinct? ... If you're thinking about adding a harness, where the fuck would you put it? If not already in the existing harnesses area, that section.

The + item is available in both the composer chat picker and New Tab picker, with the accessible name “Add harness…”. Existing model selection and harness rows remain unchanged. This is the d follow-up to #26071.

Keyboard behavior: the rail arrows reach the trailing Add action without changing the selected harness. Enter/Space keep native button activation. Returning to a harness restores focus even with a hidden search field. New Tab renders one Add action, and More models is absent when the harness has no extra models.

Validation:

• Branch base: dfdfc55265f4d5caefa8b8148618ee368382fc98 (#26589).
• Head: add512615797846ae25d1f380f8fc5bbe260a1dc.
• Focused verification: run 40657628190. Both return-from-Add regressions fail against a033e89; all three focused files pass at add512615797 without retries (model-picker-picks, QuickModelMenu, new-tab-add-harness-chip). The full webviews check passes: formatting, lint, typecheck, guards, and generated protocol.
• The empty-search keyboard repair lets ArrowDown reach Add harness and Enter activate it; returning through the current harness or directly to search clears Add so Enter selects the model; the New Tab gallery play now checks the rail action and models-only More models.
• Gallery comparison: run 40648057223 rendered the More models play in both themes. The base and head share the existing disclosure-step base red: base layout shift 0.068107, head 0.069632 in both themes (the 0.001525 increase is ~2.2%); this is recorded as a base red and the disclosure source is unchanged.

Gallery: new-tab.quick-model-menu/combined, new-tab.quick-model-menu/claude, new-tab.quick-model-menu/claude-more-models, agent-pane.model-picker/quick-menu

Before / after

Synthetic gallery captures from advisory run 40644096979, rendered at base dfdfc552 and the pre-repair visual head c588a33. The follow-up commits only repair keyboard activation and gallery assertions, so these visual states are unchanged at final head add512615797; the captures contain no private data.

State	Base light	Head light	Base dark	Head dark
Combined menu
Claude menu
Claude More models

The rail action is a + Add harness item in the head; More models contains models only. The full picker and compact picker preserve the existing menu model list.

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Summary by cubic

Keeps "Add harness…" in the harness rail so both the compact and full pickers end the strip with a clearly labelled + item instead of a row in the model list.

• "More models" now contains models only; the separate Add row is gone from the model list and submenu.
• The Add rail item is reachable via click and rail arrows; focus moves between it and neighboring harness tabs without changing the selection.
• Rail arrows reach Add even when search is hidden, empty, or the picker has no matches, with focus wrapping between Add and the Starred tab.
• Cmd-Ctrl-M over the compact menu now replaces it with the full picker instead of keeping the compact menu open.

Agent session: agent session