Replace munki-pkg with swiftpkg for managed Python packaging

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## Objective
Migrate this repository's managed Python installer packaging to [codecarton/swiftpkg](https://github.com/codecarton/swiftpkg) and retire the munki-pkg build dependency.

## Current behavior
`build_python_framework_pkgs.zsh` downloads a pinned munki-pkg commit using `MP_SHA`, `MP_BINDIR`, and `MP_ZIP`. Its `build_pkg()` function generates `recommended/build-info.json`, stages `preinstall-cleanup`, invokes `munkipkg`, and moves the signed package into `outputs/`. Notarization and stapling run separately. Without an installer identity, the script skips package creation and still produces the framework ZIP.

## Implementation scope
- Select and pin a published swiftpkg CLI release compatible with Apple Silicon build hosts; verify the downloaded artifact against a pinned SHA-256 and document prerequisites.
- Replace the munki-pkg download, temporary paths, and invocation with swiftpkg; propagate failures clearly.
- Validate compatibility of every generated build-info setting, especially `ownership`, `suppress_bundle_relocation`, `preserve_xattr`, `distribution_style`, and `signing_info`. Implement equivalent behavior for any unsupported settings.
- Preserve the package identifier (`io.macadmins.python.recommended`), version derivation, output filename/location, payload layout, permissions, symlinks, extended attributes, and preinstall cleanup behavior.
- Preserve framework code signing and the existing installer signing/notarization/stapling sequence; avoid duplicate notarization.
- Ensure Python 3.11, 3.12, 3.13, and 3.14 workflows use the replacement and continue publishing expected artifacts; update workflows only where necessary.
- Update README build prerequisites and credits. Remove active munki-pkg dependencies and obsolete bootstrap references while retaining appropriate historical attribution.

## Acceptance criteria
- [ ] Local and CI packaging use the pinned swiftpkg CLI with verified artifact integrity; no active build path downloads or executes munki-pkg.
- [ ] All generated build-info options have verified equivalent behavior, with any migration differences documented.
- [ ] On Apple Silicon, inspect a baseline munki-pkg package and replacement package to confirm equivalent receipts, version, install paths, payload, ownership/modes, symlinks, extended attributes, installer scripts, and non-relocation behavior.
- [ ] Signed packages pass signature checks, notarization, and stapling validation using the existing signing credentials.
- [ ] Clean-install and upgrade smoke tests on a disposable Apple Silicon Mac confirm preinstall cleanup, framework installation at `/Library/ManagedFrameworks/Python/Python3.framework`, the `managed_python3` symlink, and execution/imports of the managed runtime and bundled dependencies.
- [ ] The no-installer-identity path continues producing the framework ZIP without requiring signing credentials.
- [ ] Each supported Python workflow builds and publishes the expected package artifact.
- [ ] `zsh -n build_python_framework_pkgs.zsh` passes; any modified workflow YAML parses successfully.
- [ ] README documents the replacement tool, pinned version/update procedure, and prerequisites.

## Boundaries
This change replaces the package builder. It does not remove support for deploying the resulting installer through Munki, change Python/runtime dependencies, add the Swiftpkgr desktop app, or change the managed framework's installation contract.

Start with build_python_framework_pkgs.zsh, especially build_pkg(), the munki-pkg bootstrap variables, and the existing build-info.json generation. Then inspect the Python 3.11–3.14 workflow files and README prerequisites. Compare baseline and replacement packages on Apple Silicon, and use the listed shell, signing, notarization, smoke-test, and artifact checks to verify completion.

Written by the indexing model from the issue text.