feat: add flow delete/version prune and permissions assignment delete commands

[ClayChipps]

Summary

Implements docs/design/0013-flow-and-permission-set-assignment-cleanup.md: replaces three bespoke,
per-customer-repo Node scripts with proper commands in this monorepo.

• New simply-flow package:
  • sf simply flow delete — 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 — deletes obsolete Flow versions for flows found under
    --source-dir, with a --dry-run preview the original script never had.
• New command in simply-permissions: sf simply permissions assignment delete — deletes
  PermissionSetAssignments for permission sets/groups named in a destructiveChanges.xml (or
  explicit name flags), so their own destructive deploy doesn't fail or orphan assignments.
• New simply-core helper: readPackageManifestMembers(xmlContent, typeName) — shared by both
  --file-reading commands to read <members> out of a package.xml/destructiveChanges.xml-shaped
  document, instead of each command rolling its own parser.

All three commands use simply-core's existing chunkedInQuery/escapeSoqlLiteral for every SOQL
query, closing a real SOQL-injection gap the original scripts had (unescaped string interpolation into
IN (...) clauses). Every delete loop collects per-item failures into a structured result and table
instead of the originals' console.error-and-continue, and sets process.exitCode for CI gating.

The design doc has a post-implementation "Implementation notes" section covering a couple of things
the build taught: a SaveResult failure carries no id, so assignment delete's chunked delete
attributes failures by request-array position instead; and the Tooling API bulk-destroy() question
from the design doc's Open Questions is resolved as "client-library capable, server capability
unverified" — both new Flow commands keep the original's one-version-at-a-time loop rather than risk
an unverified bulk path.

One fix beyond the design doc: removed an unused @salesforce/kit dependency that got carried over
from scaffolding simply-flow off simply-permissions's package.json.

Note: pnpm-lock.yaml's diff is large but mostly cosmetic YAML-reformatting churn (same as the prior
two PRs), plus the real additions (fast-xml-parser for simply-core, glob for simply-flow).

Test plan

• pnpm run build and pnpm test pass for simply-core, simply-flow, and simply-permissions
  individually (verified independently, not just via cached wireit output)
• Root pnpm run build/pnpm test across all 16 projects passes (ran as part of the pre-push hook)
• pnpm run readme regenerated for all three packages
• New unit tests: packageManifest.test.ts (single/array normalization, missing type, empty manifest)
• New command tests: flow/delete.test.ts, flow/version/prune.test.ts,
  permissions/assignment/delete.test.ts (failure-collection behavior, --dry-run, mutually
  exclusive input flags, chunking)

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