[P2][UI-04] Portable workspace applications

Programme: #1
Epic: #19

## Objective
Turn saved workspaces into portable user-owned applications whose layouts, bindings and workflows survive agent/provider changes without exporting credentials or blanket authority.

## Scope
- Package format for schema/version, views/layouts, bindings, queries, workflow recipes, required capabilities and migrations.
- Export/import with redaction policies and artifact inclusion options.
- Rebind imported packages to compatible local connectors/accounts/capabilities.
- Compatibility report for missing/incompatible operations.
- Versioned migration and rollback.
- Optional signed package metadata for sharing/registry workflows.
- Distinguish portable definitions from non-portable source credentials/session state.

## Product rules
- Exporting a workspace never exports passwords, session cookies, secret tokens or approvals.
- Capability rebinding is semantic: identical names do not imply compatible operation meaning.
- Imported applications request fresh grants in the receiving environment.

## Acceptance criteria
- [ ] Export/import preserves the interface, filters, workflows and permitted artifacts.
- [ ] No credential/session/blanket approval material is present in exported packages.
- [ ] Import produces a clear capability/account rebinding report.
- [ ] Missing capabilities degrade to read-only/Original handoff where possible instead of fabricating support.
- [ ] Package migrations and rollback are tested across supported schema versions.
- [ ] Redacted snapshots cannot recover omitted private source material through embedded indexes/cache.

## Dependencies
- UI-01
- WS-01
- CONN-01

**First phase:** P2
**Maturity target:** P6
**Owner:** workspace-ui-data

## Task issues
- [ ] #424 `UI-04.T01` — Define portable and nonportable package fields
- [ ] #425 `UI-04.T02` — Build export selection and redaction planning
- [ ] #426 `UI-04.T03` — Implement safe archive assembly and validation
- [ ] #427 `UI-04.T04` — Implement semantic capability rebinding
- [ ] #428 `UI-04.T05` — Install definitions without activating old authority
- [ ] #429 `UI-04.T06` — Implement package versioning and rollback
- [ ] #430 `UI-04.T07` — Qualify round trips and redacted sharing

Start with task issue #424 to define the portable and nonportable fields, then review dependencies UI-01, WS-01, and CONN-01 before taking on the later task issues. The work is done when export/import preserves supported workspace content, excludes credentials and private source material, reports rebinding gaps, and validates migrations and rollback across schema versions.

Written by the indexing model from the issue text.