feat: add portable archive export and restore

[larry-zy]

Closes #1421

Rationale for this change

PowerContext needs a backend-neutral logical archive for disaster recovery, offline transfer, customer-controlled backup, and SQLite/OceanBase migration without losing immutable history, lineage, review decisions, or Handoff evidence. Database-native backup remains a separate operational recovery mechanism.

What changes are included in this PR?

• Adds a versioned .pcb contract with deterministic manifests, producer/schema versions, per-record SHA-256 digests, an aggregate checksum, deterministic ZIP output, and optional compression.
• Streams scope-selective export, inspect, target-aware dry-run validation, and transactional/idempotent restore with content-free progress.
• Preserves Scope relationships, Sources and journal positions, immutable Artifact Revisions, lineage and cross-Scope publication provenance, Memory versions/heads, Candidate decisions, Work Sources, Handoffs, and receipts.
• Requires export authorization before database enumeration, rejects credentials/provider secrets/absolute host paths, and excludes access bindings, audit/usage/evaluation data, host-local state, cursors, and projections.
• Adds immutable conflict detection, rollback-safe restore, durable readiness receipts, and Memory/Experience/Skill projection rebuilds.
• Supports configured SQLite, SeekDB, and OceanBase deployments through the local CLI; includes an opt-in live SQLite-to-OceanBase acceptance test.
• Documents format compatibility, recovery workflow, exclusions, and distinct SQLite/OceanBase native backup guidance.

Are there any user-facing changes?

Adds powercontext archive export, powercontext archive inspect, and powercontext archive restore. Restore writes require --yes; --dry-run performs target compatibility and conflict checks without domain writes. This introduces portable bundle format version 1 and the pc_portable_restore_receipts table. No existing API or bundle format is changed.

How was this change tested?

• prek run -a
• ty check
• Focused archive/schema/CLI/E2E suite: 22 passed
• make docs-test: 792 public pages and internal links verified
• Full macOS suite: 2471 passed and 101 skipped; the remaining three tests are Linux systemd-only platform guards

AI usage statement

Codex (GPT-5) was used to assist implementation, tests, documentation, and review; all changes were reviewed and validated locally.

[Teingi]

resolve conflicts