docs(pgpm): document bundle envelope, applyEnvelope, and table-data export in package READMEs

[pyramation]

Summary

Human-facing README docs for the artifact structures landed in #1433/#1435/#1436 — previously only the pgpm-migration-bundle agent skill covered them:

• pgpm/bundle/README.md: new "The bundle envelope" section — the shipping-container framing (schema + data + fixtures as one sealed, versioned, tamper-evident package), createEnvelope/verifyEnvelope example, manifest/Merkle-digest model, payload-agnostic parts, JSON + directory serialization layout.
• pgpm/core/README.md: "Migration bundles and envelopes" section — applyBundle and applyEnvelope/previewEnvelopeApply semantics (digest gate, one-transaction part replay, layered rollback plan, transformPartSql ID-remap seam).
• pgpm/export/README.md: feature entry for the generic table-data export (exportTablesData + deploy/revert/verify builders) and its role as the envelope's data/fixtures payload producer.

Docs-only; no code changes.

Link to Devin session: https://app.devin.ai/sessions/ad40d16f38a349b48eb7ce9375e27e6e
Requested by: @pyramation

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