feat(plan): carry the create-shape cause on refused statements

[Kiran01bm]

Refused create statements in a plan now carry the executor's typed CreateShapeCause, so every renderer describes the same cause instead of re-deriving it.

Why

The executor already decides why a create shape is refused (IF NOT EXISTS, PARTITION OF, a duplicate relation name, …) and returns it as a typed cause. The plan dropped that type at the refusal boundary, so the text renderer, the greenfield refusal summary, and the desired-schema path each recomputed a cause from the error. Three recomputations of one fact drift; the plan is the single place the verdict should live.

What

• plan.Statement.Cause (executor.CreateShapeCause, json:"cause,omitempty"), stamped by RefuseUnsupportedCreateShape from executor.CreateShapeCauseOf(err). First refusal wins; a refusal that carries no typed cause fails closed with ErrInvariantViolation.
• cause is explanatory: excluded from the plan fingerprint, alongside reason. Present exactly when a diff-source plan with table_exists: false refuses a statement as unsupported-statement; absent for every other refusal, including an alter-source refusal against an absent table.
• The three cause recomputations are deleted; renderers read Cause.Description().
• Plan report format_version 2 → 3; docs/plan-report.md gains a cause row, a Causes vocabulary section, and a third pinned example (greenfield IF NOT EXISTS refusal alongside an admitted index), all checked by the docs tests. The SchemaBot integration notes point the adapter at statements[].cause instead of the executor's sentinels.
• Demo tour gains a refused-diff step (IF NOT EXISTS fixture) asserting disposition, reason, and cause in check mode.

Trade-off: the duplicate-name refusal detail no longer appends the colliding name (the plan carries the typed cause only). A cause_detail field is the natural follow-up if the name proves necessary for operators.

Before / after

Before

  executor
    CreateShapeRefusals ──► []error   (typed cause inside each error)
                               │
                               ▼
  plan.Statement
    reason: unsupported-statement
    cause dropped at the refusal boundary
                               │
              ┌────────────────┼────────────────┐
              ▼                ▼                ▼
       diff_text.go         diff.go         desired.go
       re-derives cause     re-derives      re-derives
       from the error       cause           cause

After

  executor
    CreateShapeRefusals ──► []error   (typed cause inside each error)
                               │
                               ▼  CreateShapeCauseOf(err), stamped once
  plan.Statement
    reason: unsupported-statement
    cause:  if-not-exists
                               │
              ┌────────────────┼────────────────┐
              ▼                ▼                ▼
       diff_text.go         diff.go         desired.go
       Cause.Description()  — all three read the same field

[chatgpt-codex-connector]

You have reached your Codex usage limits for code reviews. You can see your limits in the Codex usage dashboard.

[chatgpt-codex-connector]

You have reached your Codex usage limits for code reviews. You can see your limits in the Codex usage dashboard.