document the per-node execution model; concept.md is stale (ON CLUSTER, chschema --auto-approve)

Symptom

The execution model is per-node: generated DDL is deliberately run on each node of a cluster individually, never with ON CLUSTER. But this model is documented nowhere, and the existing docs say the opposite or reference commands that don't exist:

• docs/concept.md:20 — "Cluster-aware execution. Generated DDL uses ON CLUSTER where appropriate." (stale; the project moved away from ON CLUSTER because it caused too many problems)
• docs/concept.md:36-41 — workflow references chschema validate ... live ClickHouse apply, chschema --dry-run, and chschema --auto-approve, none of which exist in hclexp.
• There is no documented path from diff -sql / plan -format json output to actual per-node execution (who runs it, how nodes are enumerated, ordering guarantees).

Related inconsistency in code: createTableSQL still emits ON CLUSTER when a table has cluster set (internal/loader/hcl/sqlgen.go:627-630) while ALTERs never do — if per-node execution is the rule, the CREATE-side emission is the leftover to remove or gate.

Fix direction

• Rewrite docs/concept.md to describe the actual model: per-node execution, no ON CLUSTER, plan/diff -format json as the hand-off to the executor.
• Document the end-to-end apply workflow (even if execution lives in external tooling).
• Decide the fate of the ON CLUSTER emission in createTableSQL and make CREATE/ALTER consistent.

Found in the 2026-07-02 deep-dive audit (docs/plans/2026-07-02-deep-dive-improvement-areas.md, C1); reframed after maintainer clarified the per-node execution policy (see #81).

Start with docs/concept.md and the execution policy clarified in #81, then inspect the plan and diff command references. Review createTableSQL in internal/loader/hcl/sqlgen.go:627-630 and compare its ON CLUSTER behavior with ALTER generation. Done means the documentation describes per-node execution and the CREATE/ALTER behavior is made consistent.