docs(cli): add register help examples

[corylanou]

Description

This stacked draft PR adds an Examples: section to litestream register -help and adds a regression test that locks those examples into the usage output.

This PR is intentionally scoped to the register checkbox only.

Motivation and Context

Issue #1232 tracks agent-friendly CLI improvements, including runnable help examples for command discovery.

Related to #1232
Base branch: #1236 (feat/1232-required-flag-try-hints)

How Has This Been Tested?

• go test -run 'TestRegisterCommand_Run|TestRegisterCommand_Usage' ./cmd/litestream
• go test ./cmd/litestream
• go test ./...
• pre-commit run --all-files

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (would cause existing functionality to not work as expected)

Checklist

• My code follows the code style of this project (go fmt, go vet)
• I have tested my changes (go test ./...)
• I have updated the documentation accordingly (if needed)

[github-actions]

PR Build Metrics

✅ All clear — no issues detected

Check	Status	Summary
Binary size	✅	35.97 MB (+4.0 KB / +0.01%)
Dependencies	✅	No changes
Vulnerabilities	✅	None detected
Go toolchain	✅	1.25.9 (latest)
Module graph	✅	1206 edges (0)

Binary Size

	Size	Change
Base (cf496ee)	35.96 MB
PR (e7f6906)	35.97 MB	+4.0 KB (+0.01%)

Dependency Changes

No dependency changes.

govulncheck Output

=== Symbol Results ===

No vulnerabilities found.

Your code is affected by 0 vulnerabilities.
This scan also found 1 vulnerability in packages you import and 0
vulnerabilities in modules you require, but your code doesn't appear to call
these vulnerabilities.
Use '-show verbose' for more details.

Build Info

Metric	Value
Build time	42s
Go version	go1.25.9
Commit	e7f6906

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_{🤖 Updated on each push.}