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

[easonLiangWorldedtech]

Part 1/8 of the task-tree series.

Description

Adds a validated depth to history items and a cycle-safe backfillTaskDepth() that propagates parent depth through the delegation tree, so every task knows its nesting level. Depth is surfaced for later use by settings validation, environment details, and history-tree display (later parts of this series).

Contract rules:

• A persisted depth on the task itself is authoritative.
• Otherwise the live parent's depth + 1 is used; the child inherits the parent's authority so a legacy non-authoritative parent never stamps its placeholder depth onto the child as a persisted fact.
• Cycle, dangling-reference, and over-long chains resolve to undefined (treated as root) instead of persisting a bogus value.

Includes the single-open-invariant spec mock for backfillTaskDepth so unit tests pass standalone.

Test Procedure

• pnpm --dir src exec vitest run core/task/__tests__/taskDepth.spec.ts core/task/__tests__/Task.spec.ts __tests__/single-open-invariant.spec.ts
• New regression cases: non-authoritative parent inheritance (Task.spec.ts), exactly-MAX_DEPTH_WALK boundary (taskDepth.spec.ts).

Pre-Submission Checklist

• Scope: one concern per PR (depth contract only)
• Self-review performed
• Tests added for new behavior (regression cases above)
• Visual snapshot: not applicable (no UI change)
• Documentation impact: none required