docs: describe install namespace migration

[tmshort]

Summary

Updates the migration plan, requirements, validation matrix, and E2E guide for install-namespace migration.

• Documents target namespace preparation, resource handoff, and source cleanup.
• Describes the explicit namespace-deletion acknowledgment.
• Records the remaining system-managed namespace requirement gap.
• Adds the fixture cross-namespace test to the documented run order.

Depends on

• migration-install-namespace-tests

Validation

• git diff --check

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📥 Commits

Reviewing files that changed from the base of the PR and between 32313cf and 4f3dc22.

📒 Files selected for processing (4)

• specs/20260821-migration-v0-to-v1/e2e.md
• specs/20260821-migration-v0-to-v1/plan.md
• specs/20260821-migration-v0-to-v1/requirements.md
• specs/20260821-migration-v0-to-v1/validation.md

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