feat(config): span-annotated validation errors via miette

[aram-devdocs]

Summary

Validate config inputs with miette-style span annotations so
errors pinpoint the offending line in plumb.toml.

Crate / area

plumb-config — figment loader + schemars schema emission

Phase / batch

Part of #20 → Batch 2A (Config shape — remaining sections + span-annotated errors).

Effort: M

PRD references

• PRD §12.4

Acceptance criteria

• Bad palette values yield miette diagnostic with span.
• Unknown keys yield miette diagnostic.
• Integration test covers 4 failure classes.

Dependencies

• None (first in batch; batch-level deps gate this).

Review gates

• 02-spec-reviewer
• 03-code-quality-reviewer
• 05-architecture-validator
• 04-test-runner

How to deliver

Dispatch /gh-issue <this-issue-number>. See the root AGENTS.md and the scoped plumb-config/AGENTS.md for the invariants that apply.

Suggested delivery: gh-issue