fix(rules): clarify null-field remediation

[pablontiv]

What

Clarify the error emitted when a child .stem tries to remove an inherited schema field with null.

The message now distinguishes scoped remedies—narrowing the declaring stem's scope.match or moving records outside its scope—from removing the declaration globally. The Rootline skill carries the same guidance.

Related issue

Closes #200

Why

The previous message named only global removal. For the common subtree-scoped case, that advice could remove governance from every record beneath the declaring stem instead of only the affected records.

How

• Preserve the existing null-field rejection and diagnostic shape.
• Name both scoped exits and the global exit in the ParseStem error.
• Add a direct regression test against the real parser behavior.
• Synchronize .claude/skills/rootline/SKILL.md without including stem health reports a semantic .stem refusal as "yaml-valid", with an empty field and an absolute path #199 diagnostic metadata changes.

Checklist

• Tests pass (go test ./... -race via just test)
• Code is clean (go vet ./...; just check reports 0 lint issues and builds successfully)
• Changes are documented if user-facing
• Linked to its issue with a closing keyword, or deliberately left unlinked (intermediate PR of a chain)

Additional verification: just coverage-check passes at 90.5% total coverage.