fix: resolve remaining merged-PR review follow-ups

[rmanibus]

Summary

• apply still-relevant follow-ups from merged PR review comments across #61, #72, #74, #83, #84, and #86
• harden source behavior for edge cases: incremental delete parent fallback, Google Drive nil change payloads, root-relative path handling, OneDrive root-path normalization/encoding, and cleanup of accidental test artifact files
• tighten CLI/profile/completion behavior (store URI validation, boolean flag completion parsing, deep-clone missing global flags), fix pack-store logging race, and align benchmark/check scripts with current password/env behavior

Validation

• go test -count=1 ./pkg/source
• go test -count=1 ./cmd/cloudstic
• go test -count=1 ./...
• golangci-lint run ./...

[codecov]

Codecov Report

❌ Patch coverage is 59.09091% with 45 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/source/local_source_linux.go	37.50%	19 Missing and 1 partial ⚠️
pkg/source/onedrive.go	60.71%	9 Missing and 2 partials ⚠️
internal/engine/backup_scan.go	44.44%	5 Missing and 5 partials ⚠️
cmd/cloudstic/cmd_profile.go	40.00%	3 Missing ⚠️
pkg/source/onedrive_changes.go	87.50%	1 Missing ⚠️

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