🤖 feat: persist explicit AI-setting changes from the VS Code webview

[ThomasK33]

Problem

The VS Code webview never saves AI-setting changes to the workspace. Sends pass skipAiSettingsPersistence: true (#4765), and a model change stays local. After #4778 the webview loads each workspace's real settings, but a user's explicit model, thinking or agent choice made in the webview still does not carry over to the desktop app or to later sessions.

Turning persistence on safely needs the desktop's send-path handling. Codex review of #4778 listed what is missing:

• Selection intent for sub-agent workspaces: getAiSelectionIntentForSendOptions, with the tokens consumed after a successful send, so deliberate picks update taskAiPins.
• Gateway routes: persist a gateway-preserving model (normalizeSelectedModel), not the canonical direct-provider ID.
• Unclamped thinking: persist the selected thinking level as is; the backend applies the configured floor.
• Write order: serialize or supersede updateAgentAISettings writes so rapid picks persist in selection order.

Proposed fix

Reuse the desktop's send-options and selection-intent helpers in the webview composer, rather than copying their logic, and enable persistence only for workspaces with loaded settings.

Refs #4738

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[linear-code]

MUX-294

[ThomasK33]

Picked up by the Issue fix coordinator's workspace-25 (plan approved).

Approach: persist explicit webview picks at send time through the desktop's own path (maybePersistAISettingsFromOptions plus aiSelectionIntent), gated on loaded settings, a policy-allowed stored model, no unresolved persisting send, and an explicit still-current pick. No pick-time updateAgentAISettings writes.

Delivery, sequential:

1. PR 1 (branch fix/webview-model-pick-semantics): webview model picks keep gateway IDs and record selection intent; sends still never persist.
2. PR 2 (branched from main after PR 1 merges): the gated send-time persistence and its ordering guarantee.

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[ThomasK33]

Done: #4834 (61dc6ca) and #4836 (32873c8) merged. The VS Code webview now saves explicit model and thinking picks at send time, through the desktop's own path.

Residuals, not tracked unless requested (all fail closed or match desktop behavior):

• A failed policy.get reads as "no policy" in the shared PolicyContext, so a pick made then can be saved. The backend still enforces policy on every turn, and the desktop saves regardless of policy.
• The webview's persistence guard is keyed by workspace ID only. A workspace ID that exists on two servers could skip saves until reload.
• On a main workspace, a model-only pick saves the per-load snapshot of the thinking level (desktop parity).
• Agent-only toggles (Exec/Plan) are not saved on their own (scope decision D3).

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