docs: add design for flow/permission-set-assignment cleanup

[ClayChipps]

Summary

Design doc for replacing three bespoke, per-customer-repo Node scripts (deleteFlows.mjs,
deletePermissionSetAssignments.mjs, deleteObsoleteFlowVersions.mjs) with proper commands in this
monorepo:

• sf simply flow delete — new simply-flow package. Deactivates and hard-deletes every version of
  the Flows named in a destructiveChanges.xml (or via explicit --flow-name flags) — required
  before a destructive metadata deploy can remove a Flow that still has an active version.
• sf simply flow version prune — same new package. Deletes obsolete Flow versions for flows found
  under --source-dir, with a --dry-run preview the original script never had.
• sf simply permissions assignment delete — new command in the existing simply-permissions
  package. Deletes PermissionSetAssignments for permission sets/groups named in a
  destructiveChanges.xml, so their own destructive deploy doesn't fail or orphan assignments.

Also proposes a new simply-core helper, readPackageManifestMembers, so both --file-reading
commands share one parser for the <types><name>/<members></types> shape instead of each rolling
their own — and fixes a real SOQL-injection gap in the originals (unescaped string interpolation into
IN (...) clauses) by reusing simply-core's existing chunkedInQuery/escapeSoqlLiteral.

This is a design-only PR — no implementation yet. Rationale for the package split (new simply-flow
vs. folding into simply-cicd) and the flow delete vs. flow version prune naming asymmetry are both
recorded in the doc's Alternatives Considered section.

Test plan

N/A — documentation only, no code changes.

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