Remove deprecated adapter shims (IntrinsicAdapter/EmbeddedIntrinsicAdapter/CustomIntrinsicAdapter) — PR2 of #1144 #1621

Parent: #1144 (PR2 of the two-PR split described there)
Depends on: #1619 (PR1 — composed-Adapter-direct path) merging first, with
its own green CI on main as the actual proof nothing still needs the shim
path.

Goal

Remove IntrinsicAdapter, EmbeddedIntrinsicAdapter, and
CustomIntrinsicAdapter (mellea/backends/adapters/adapter.py) now that
#1619 makes the composed Adapter (Identity + IOContract + a weights
binding) a fully working, additive replacement for every backend. Not urgent
— PR1 changes nothing for existing callers, so this can land whenever a
maintainer picks it up.

This issue exists so the deletion has a home to be scheduled against,
distinct from #1144 (which also tracks PR1). See #1144's "PR2 — delete the
shims" section for the full itemized scope (files, tests to delete, the
required full-repo reference sweep, and acceptance criteria) — not
duplicated here to avoid the two drifting out of sync.

Summary of scope (see #1144 for detail)

• Delete the three shim classes and their re-exports in
  mellea/backends/adapters/__init__.py.
• Remove the now-dead isinstance branches in LocalHFBackend/
  OpenAIBackend that feat(backends): accept composed Adapter directly (Epic #929, issue #1144, PR1) #1619 made redundant.
• Delete the shim-specific test files/cases (not anything that ended up
  covering the composed path instead).
• Full-repo sweep (gh api search/code for all three class names) — zero
  hits outside changelog/historical text.
• Changelog entry with a migration note (import path change for anyone
  still constructing a shim directly).

Not before

Do not start this until #1619 has merged and a subsequent CI run on main
confirms the composed path alone is sufficient — that run is the acceptance
gate, not a comment claiming it.