feat(kun): add Kun CLI usage provider

[Legiani]

Summary

Adds Kun CLI (kun) as a 13th local usage source. Kun persists session usage in an SQLite database under ~/.kun/data/index.sqlite3 in the usage_events table with (thread_id, seq, timestamp, model, usage_json).

Key details:

• Per-turn delta computation: Kun records cumulative session stats in usage_json (promptTokens, completionTokens, cachedTokens, totalTokens, costUsd); the reader calculates per-turn token and cost deltas across sequential events in each thread.
• Durable roots & environment: Default root is ~/.kun/data/index.sqlite3 with KUN_DATA_DIR override support registered in UsageEnvironment, CustomScanRoots, and tests.
• Extension contract compliance: Implements UsageProvider and registers in UsageStore.init providers: array without provider-specific logic in generic paths.

Type of change

• New feature

UI changes

No UI structure changes. Users with a Kun database get a Kun capsule in the provider tab bar and an entry in Settings → Additional scan folders; Kun token usage and cost join today's aggregate totals.

Checklist

• swift build and swift test pass locally (907 tests, 0 failures)
• PR title and description are written in English
• Tests added in KunUsageTests.swift and registry tests updated

[chattymin]

Please preserve already observed usage when Kun threads disappear. Merge cache entries by a stable entry ID, keep the larger cumulative value for an existing thread, and retain previously observed entries that are absent from a later scan instead of replacing the cache wholesale. Please add a regression test proving that deleting a thread cannot reduce historical totals, update the data-source documentation, and rebase onto current main.