[codex] Avoid overwriting same-second migrations

[KeKs0r]

Summary

Fixes a flake in the refreshable materialized view lifecycle test by preventing generated migration files from being overwritten when multiple chkit generate runs happen within the same second.

Root Cause

Automatic migration filenames used second-level timestamps plus the default auto name. The lifecycle test generates several different migrations quickly, so a later generate could reuse the same filename and overwrite an already-applied migration. The next migrate then failed correctly with a checksum mismatch.

Changes

• Write migration files using exclusive create semantics.
• On filename collision, fall back to deterministic suffixes like _001, _002, etc.
• Add a regression test that forces two migrations to share the same timestamp and verifies the first file is preserved.

Validation

• bun test packages/codegen/src/index.test.ts
• bun run --cwd packages/codegen typecheck
• bun run --cwd packages/codegen lint
• bun run --cwd packages/cli typecheck
• doppler run --project chkit --config ci -- bun test packages/cli/src/clickhouse-live.e2e.test.ts -t "refreshable materialized view lifecycle" --timeout 240000
• Looped the live lifecycle test 12 times after the fix; all 12 passed.