feat(task): track task nesting depth with cycle-safe backfill

[easonLiangWorldedtech]

Summary

Part of the Task Tree plan (issue #9, upstream Zoo-Code-Org#856). PR-1 introduces nesting depth tracking for tasks — the foundation for maxNestingDepth enforcement and auto-flatten in later PRs.

Changes

• packages/types/src/history.ts: optional depth: z.number().int().min(0) on HistoryItem (root = 0). Optional so legacy records stay valid.
• packages/types/src/task.ts: CreateTaskOptions.depth? and TaskLike.depth.
• src/core/task/Task.ts: readonly depth + depthAuthoritative. Constructor precedence: persisted value > live parent (parent.depth + 1) > root (0). A legacy child resumed without its live parent is marked non-authoritative so a placeholder 0 is never written back to history.
• src/core/task-persistence/taskMetadata.ts: persists depth only when a valid number is supplied.
• src/core/webview/ClineProvider.ts: backfillTaskDepth() — on createTaskWithHistoryItem, tasks the constructor could not resolve walk the parentTaskId chain through the in-memory store / global state and persist the computed depth (bounded, cycle-safe).
• src/core/task/taskDepth.ts (new): pure computeTaskDepth(taskId, ownDepth, lookup) helper — handles persisted values, ancestor depths, roots, cycles, dangling refs, and over-long chains.

Tests

• New unit suite core/task/__tests__/taskDepth.spec.ts (8 cases: persisted value, invalid depth fall-through, root derivation, nearest-ancestor depth, cycle, dangling ref, over-long chain, self-reference).
• Task.spec.ts: 4 new constructor cases (root=0 authoritative; child = parent+1; persisted beats live parent; legacy child without live parent is non-authoritative).

Verification

• pnpm run check-types — all 14 packages pass.
• Narrowest suites: taskDepth.spec.ts (8/8), Task.spec.ts (96/96), taskMessages.spec.ts (4/4), provider rehydration specs ClineProvider.flicker-free-cancel.spec.ts + checkpointRestoreHandler.spec.ts (17/17).
• ESLint clean on all touched files; no suppression count changes.

Notes

• No behavior change for existing tasks: depth is additive and optional everywhere; saves only include it when authoritative.