[Task] Write-Ahead Log persistence (atomic snapshot + boot restore)

[arasHi87]

Description

Survive agent crash with continuity and hard reboot with ≤60s loss: atomic tmp+rename WAL with .bak rotation, schema-version envelope, u64-as-string encoding, 60s flush ticker, boot-time restore before the first scrape.

Acceptance Criteria

A detailed list of conditions that must be met for the story to be accepted. These statements must focus on functional behavior and constraints rather than specific code-level implementation.

• Atomic tmp+fsync+rename, .bak fallback
• schema_version envelope; ,string u64 encoding
• Boot restore seeds GlobalState before scrape
• WAL health metrics

Story Points: 3

Points	Effort / Resources	Description
1	Low	Low complexity; can be resolved easily.
3	Medium	Moderate workload; requires some focus, but risks are well-managed.
5	High	High complexity; difficult to implement, requiring significant time and effort.
8	Very High	Extreme complexity or high uncertainty. If possible, this should be divided into multiple user stories.

Notes

Delivered in PR #10. Sprint 2 (2026-05-15).