Document deprecated dashboard fields in the create-dashboard skill

[terzioglub]

Summary

The bundled create-dashboard skill had drifted behind the schema-unification breaking changes. It still taught two authoring forms that no longer validate:

• Chart x: region / y: [revenue] (bare column names) — x/y now require axis encoding objects ({ field: ... }).
• file: queries/revenue.sql query references — removed; queries are inline sql: or a named query: (TSX include() still works).

This PR:

1. Fixes the stale chart example to use x: { field: region } / y: { field: [revenue] } and corrects the query-reference guidance.
2. Adds a "Deprecated Fields" section mapping every removed field to its current replacement and instructing the agent to refactor any of them it encounters in an existing dashboard (preserving column/formatting/labels, then re-validating).

Deprecated → replacement

Deprecated	Replacement
Chart x: col / y: [col]	x: { field: col } / y: { field: [col] }
file: path.sql	inline sql: or named query: (TSX include())
Metric column/prefix/suffix/format	value: { field, type, format }
Dashboard inline semantic: block	separate semantic/*.yml + model:

Grounded in the actual schema/loaders, not assumptions — confirmed include() is still supported (pkg/dashboard/jsloader.go), so it is not listed as deprecated.

Testing

• make test
• validated the corrected chart example with dac validate (passes)
• confirmed docs/ already teaches the correct forms (no stale docs)

Checklist

• scanned docs/ — already current; no changes needed
• examples updated if the authoring model changed (n/a — fixing skill to match existing model)