Design: new `func pack` experience & az CLI integration

Redesign the `func pack` experience for v5 and define how it integrates
with `az functionapp` deployment paths. Covers: which artifact shapes
`pack` produces (zip / oci / etc.), per-language behaviour
(dotnet / node / python / powershell / go / custom handlers),
relationship to `func publish`, and the contract `az` consumes.

## Definition of Done

- [ ] Design doc landed in `docs/proposed/` covering: CLI surface,
artifact shapes, per-language behaviour, relationship to
`func publish`, and the `az` integration contract.
- [ ] Decision recorded on per-language pack subcommands (separate vs
auto-detect).
- [ ] Follow-up issues opened for each implementation slice the design
surfaces.
- [ ] Doc reviewed by `az` Functions team + at least one PM.

Start by surveying the current `func pack` and `func publish` experiences and the `az functionapp` deployment paths before drafting a document in `docs/proposed/`. Done means the design covers the requested CLI, artifact, language, publish, and `az` contract decisions, records the subcommand choice, opens implementation follow-ups, and receives the specified review.

Written by the indexing model from the issue text.