feat(telemetry): export and aggregate fleet evidence snapshots

[pacphi]

Summary

Add a vendor-neutral fleet evidence contract so operators can export consistent usage and retained maintenance data and aggregate repeated snapshots without double counting.

• Add ak telemetry export, validate, aggregate, schema, and metrics.
• Define strict versioned schemas, private installation identities, allowlisted fields, separate observed/estimated costs, source coverage, freshness, and latest-snapshot replacement.
• Bound retained inventory reads and decompression; keep telemetry offline with clean JSON output and no-clobber file writes.
• Document the contract, operational limits, implementation plan and ADR-0054.

Validation

• 56 focused telemetry tests passed.
• Full local suite: 4,230 passed, 6 existing skips; legacy suites passed.
• Typecheck, lint, complexity, markdown and build/package checks passed.
• Branch CI passed all 13 jobs on commit 582d552: Linux/macOS/Windows on Node 22/24/26, minimum runtime, quality/audit, UI and internal links.
• Independent review findings have regression tests and fixes. AQE's inconsistent executor response is not counted as passing evidence.

Scope

Offline snapshots and a reference reducer; no vendor dashboards, upload service or scheduler. Usage contains whole retained sessions selected by end time, not disjoint billing windows. Transcript copies on different installations remain distinct observations. Content digests are integrity checks, not sender authentication.