Add pkg/lint: offline typed findings replacing the CLI lint stub

[Kiran01bm]

Summary

Replaces the CLI lint stub with a real offline linter: pkg/lint runs the same parse-and-classify pipeline as the front doors but with zero live facts, so it needs no database and is strictly conservative. Findings carry typed codes automation branches on — never prose. Second slice of the P2.5 dry-run/advisory surface, stacked on the plan-contract PR.

What

• New pkg/lint: Check(script) returns a versioned Report (format_version: 1) of typed findings. Codes map from the classifier:
  • unsupported-operation — the engine would refuse it (error; the only severity that flips the exit code)
  • blocking-idiom — a safer native form exists; the finding carries it as suggestion (warning)
  • table-rewrite — needs the copy-and-swap path, with the planner's typed reason (warning)
  • destructive — column/constraint drops, same definition as the declarative differ (warning)
• New statement.Split: grammar-backed script splitter returning canonical per-statement SQL — no hand-parsing; the coming suggest surface reuses it.
• CLI lint [file] (or stdin), --json, offline — no DB flags. A clean script prints nothing and exits 0; error findings exit non-zero via a typed sentinel.
• An unsupported operation is a finding, not a lint failure — one bad statement never hides the rest of the report. A parse failure is an error, surfaced.
• Docs swept: README status → Phases 1–2.5; SAFETY.md, architecture package map, and low-level design stub markers flipped; testing matrix rows added.

Why

The linter is the policy gate the tracker's E1 slice needs before execution: refuse what the engine cannot run safely, and surface what it would rewrite or gate, in CI, without touching a database. Deriving findings entirely from the classifier keeps one source of truth for safety judgment — the linter adds severity and presentation, never a second opinion.

Before / after

Before:
  pg-sprite lint ──> "lint: not implemented yet (Phase 0 stub)"

After:
  pg-sprite lint change.sql
    ├─ parse (statement.Split, real grammar)
    ├─ classify per statement (planner, zero facts — conservative)
    └─ typed findings: errors → exit non-zero, warnings advise
  pg-sprite lint --json ──> lint.Report  format_version: 1

References

• PLAT-38440 (P2.5: dry-run plan + advisory suggest surface + linter)
• Stacked on the plan-contract PR (kiran01bm/p2-5-plan-contract)

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