feat: add source discover command

[rmanibus]

Summary

• add a new source discover command as the first RFC 0014 rollout slice
• support human-readable output, json output, and portable-only filtering
• discover local source candidates with mount metadata and portable-drive hints
• wire help text, completion, README, and user-guide docs

Testing

• env GOCACHE=/tmp/cloudstic-gocache go test ./pkg/source
• env GOCACHE=/tmp/cloudstic-gocache go test ./cmd/cloudstic

Related to #130
Closes #192

[codecov]

Codecov Report

❌ Patch coverage is 85.36585% with 30 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cmd/cloudstic/cmd_source.go	73.91%	9 Missing and 3 partials ⚠️
internal/engine/source_discover.go	83.67%	6 Missing and 2 partials ⚠️
internal/engine/source_discover_linux.go	80.48%	7 Missing and 1 partial ⚠️
cmd/cloudstic/main.go	0.00%	2 Missing ⚠️

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