docs: Align bundle v1 with S3 exporter and cutover support

[acburdine]

Summary

Aligned the unpublished bundle v1 contract with the S3 exporter in Ghost-CLI #2333: required metadata, top-level kind, ghost.version, explicit database/member/content paths, and public/admin URLs. Added matching MySQL and portable manifest fixtures.

Documented --leave-stopped, original-state recovery, private output handling, and portable fidelity boundaries. Updated the plan to keep local SQLite portable capture sequential (content API → members CSV → stop → assets), with no write-freeze machinery or Ghost changes. Configuration remains .env plus ghost.env.

Verification

• node --test --test-timeout=120000 tests/*.test.mjs: 240 passed.
• Cross-repository fixture comparison and real Compose container round trip passed in Ghost-CLI's suite, using this repository's scripts/lib/env.sh serializer.
• Ghost-CLI full suite: 1,087 tests passed; lint passed.

Scope

Documentation and fixtures only. Docker import and end-to-end destination identity/member/subscription reconciliation remain S5 work. No importer, backup/restore, upgrade or supervision implementation.

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