docs(release): Prepare 1.0.0-beta.5

[nfebe]

The release carries sourceant update, sourceant start, and commands that start the agent themselves on a free port. beta.4 is already published, so the beta counter moves.

[sourceant]

Review complete. See the overview comment for a summary.

[sourceant]

Code Review Summary

Prepares the 1.0.0-beta.5 release: VERSION moves from beta.4 to beta.5, and CHANGELOG.md documents sourceant update, sourceant start, commands that start the agent themselves on the next free port, and sourceant setup starting what it installed. The Go changes across internal/agent/client.go, internal/command/lifecycle.go, internal/command/root.go, and internal/update/update.go only shorten documentation comments; no behavior changes.