feat(plan): disclose greenfield steps as plain executable statements

[Kiran01bm]

Greenfield plan statements now disclose what the create path actually executes: exec_sql is the statement as written, each build's decision is reclassified metadata-only, and no CONCURRENTLY substitution is shown for a table that does not exist yet.

Why

A greenfield CREATE TABLE plan (the table does not exist yet) is executed statement-by-statement by the create path, which runs the desired file's builds verbatim and refuses a concurrent build on a table born this run. The plan report disclosed the opposite: the planner's live-table rewrite (CREATE INDEX CONCURRENTLY …) as the greenfield exec_sql, with a safer-idiom decision and a blocking-lock warning in the human report. An orchestrator that ran exec_sql itself ran a form the engine never would, and a reviewer was told a table nobody reads yet was about to be locked.

What

• plan.DiscloseGreenfieldExecution sets exec_sql to the statement itself and execution: autocommit for every executable greenfield statement, withdraws safer_sql, and re-stamps a safer-idiom decision to metadata-only — the reason the planner already gives the CREATE TABLE itself. Refused, rewrite-required and unavailable statements are untouched.
• diffplan calls it on the greenfield branch, so pg-sprite diff --json, --sql (-- native (metadata-only)), the human report (note[metadata-only] … runs as written) and the library Report all agree.
• Withdrawing per-decision advice is one helper shared with RefuseUnsupportedPartitionedParent.
• Docs: plan-report.md (field rows, reason vocabulary, Fingerprint), postgres-online-ddl-reference.md (metadata-only / safer-idiom), safer-sequences.md, schemabot-integration.md.

Fingerprint change, no version bump. plan.Fingerprint hashes exec_sql, so every greenfield plan's fingerprint changes value (for the same desired file, sha256:63a6cd9e… → sha256:df731940…). The serialization is unchanged and format_version stays 2; the fingerprint now commits to what runs. A consumer holding a pre-upgrade greenfield fingerprint gets one plan-fingerprint-mismatch and re-plans. Fingerprints for existing tables are unaffected. docs/plan-report.md carries the caveat.

Before / after

Before (greenfield CREATE INDEX statement)      After
+------------------------------------------+    +------------------------------------------+
| sql:        CREATE INDEX i ON t (c)      |    | sql:        CREATE INDEX i ON t (c)      |
| exec_sql:   CREATE INDEX CONCURRENTLY …  | -> | exec_sql:   CREATE INDEX i ON t (c)      |
| execution:  autocommit-each-step         |    | execution:  autocommit-each-step         |
| reason:     safer-idiom                  |    | reason:     metadata-only                |
| safer_sql:  CREATE INDEX CONCURRENTLY …  |    | safer_sql:  (absent)                     |
| text:       warning[safer-idiom] …       |    | text:       note[metadata-only] …        |
|             will run a safer online seq. |    |             runs as written              |
+------------------------------------------+    +------------------------------------------+
   (create path refuses CONCURRENTLY here)         (what the create path runs)

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