feat(settings): add maxNestingDepth and autoFlattenOnLimit settings round-trip

[easonLiangWorldedtech]

feat(settings): add maxNestingDepth and autoFlattenOnLimit settings round-trip

Part of the Task Tree unified subtask management plan (issue #9, P1.2). This PR wires two new persisted settings through the complete AGENTS.md round trip so that later PRs (auto-flatten inline mode) can read them from both the extension host and the webview.

Settings

Setting	Type	Default	Range / semantics
maxNestingDepth	number	2	integer 0–5; root = 0. 0 disables delegation entirely (all subtasks run inline).
autoFlattenOnLimit	boolean	true	when the nesting limit is reached, execute the subtask inline in the current conversation instead of opening a child tab. When false, such calls are rejected so work continues directly.

Round-trip (per AGENTS.md persisted-setting checklist)

1. Schema + defaults — packages/types/src/global-settings.ts: DEFAULT_MAX_NESTING_DEPTH = 2, DEFAULT_AUTO_FLATTEN_ON_LIMIT = true; schema fields maxNestingDepth: z.number().int().min(0).max(5).optional() and autoFlattenOnLimit: z.boolean().optional(). Old data stays compatible (both optional).
2. ExtensionState — packages/types/src/vscode-extension-host.ts: both fields added as optional.
3. SettingsView binding — webview-ui/.../SettingsView.tsx destructures from local cachedState (not live state), includes both in the handleSubmit() payload, and passes them to ContextManagementSettings, which renders a 0–5 slider (max-nesting-depth-slider) and a checkbox (auto-flatten-on-limit-checkbox) with i18n keys under settings:taskTree.*.
4. Persistence + normalization — webviewMessageHandler.ts: maxNestingDepth is normalized to an integer clamped 0–5 (non-numeric falls back to the default); autoFlattenOnLimit is coerced to a boolean; unset values are skipped so they never clobber stored state. Final value persisted via generic contextProxy.setValue().
5. Provider round trip — ClineProvider.getState() returns both with defaults (?? DEFAULT_...); getStateToPostToWebview() destructures and passes them through, completing storage → webview so a saved control never visually reverts.

Tests (true / false / unset covered)

• webview-ui/.../ContextManagementSettings.spec.tsx: slider renders default 2 when unset, explicit value passthrough, change callback; checkbox checked by default, unchecked when explicitly false, toggle callback.
• src/core/webview/__tests__/ClineProvider.spec.ts (taskTree settings round-trip): getState() defaults (unset → 2 / true); handler clamping (3→3, 9→5, -1→0, non-numeric→default) and skip-when-unset; boolean normalization (false stays false, truthy coerces to true) and skip-when-unset; getStateToPostToWebview() saved-value passthrough and defaults when unset.

Verification

• pnpm run check-types — 14/14 packages pass
• ContextManagementSettings.spec.tsx — 35/35 (incl. 6 new)
• ClineProvider.spec.ts — 132/132 (incl. 4 new round-trip tests)
• SettingsView.change-detection + SettingsView.unsaved-changes — 15/15
• Full turbo lint (--max-warnings=0) clean; no eslint suppression count increases

Out of scope (later PRs)

Runtime consumption of these settings (auto-flatten inline mode, depth enforcement in new_task) lands in the follow-up PRs per issue #9. This PR is purely the settings plumbing.