docs: uf init warns on stale command references after namespace migration

[yvonnedevlinrh]

Context

PR unbound-force/unbound-force#420 hardened the uf init command rename migration with warnings.

What changed

• cleanupRenamedCommands() now emits warnings on file removal failures (previously silent)
• New warnStaleCommandRefs() scans agent markdown files for references to old-name commands (pre-uf.* namespace) and warns users
• Users see actionable output like: "Agent file coordinator.md references stale command /review-council — should be /uf.review-council"

What needs updating

• Document the stale reference warning behavior in the uf init command reference
• Mention this as part of the namespace migration story (related to website#220)
• Note that running uf init on existing repos will surface these warnings

Affected pages

• content/docs/reference/cli.md — uf init command reference
• Potentially a migration guide if one is created for the uf.* namespace change