docs/cleanup: sweep decommission-residue citations (68 ADR-00NN refs, 19 cognee refs) + retire the cognee memory engine enum

[thinmintdev]

After the big decommissioning waves (Lemonade→container runtime #687, Cognee→Hindsight ADR-0023, chat/primary→agent/utility, hardened-perms removal), the code comments and one config enum were never swept. docs/internal/ (the ADRs) is gitignored (#638), so every in-code docs/internal/adr/00NN-*.md citation is a dangling reference.

Scope

• ~68 ADR-00NN citations in src/**/*.py docstrings/comments pointing at ADR files that are no longer tracked (e.g. ADR-0006 §7 in config/schema.py, capabilities/config.py, registry/model_meta/__init__.py; ADR-0012/0015/0020 in api/openrouter/*). Repoint to ADR-0023 / inline the surviving decision, or drop the dead path.
• ~19 cognee references in src/**/*.py (e.g. mcp/memory.py:1-8, schema.py:1913-1918 pointing at the removed cognee_wrapper.py; memory/migrate.py).
• Memory engine enum still lists/defaults to the removed store: config/schema.py:2049-2062 keeps the engine doc "Set to cognee to revert to the untouched Cognee store" and the validator still accepts cognee (resolves to Hindsight anyway). Drop cognee from the known set + description and change the validator fallback.
• Misc stale module docstrings: providers/kokoro.py:20-23 (:ro smuggling), openwebui/__init__.py:5+env_writer.py:4 (a Settings-API caller that does not exist), registry/detect.py:82 (advertises a rerank cap it never emits).

Acceptance criteria

• No src/** comment cites a docs/internal/adr/00NN path that is not tracked
• No live cognee references in src/** except intentional historical-changelog text
• engine enum no longer offers cognee; validator + tests updated

Complements (stub-era comments) #776.

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Filed from the v0.8.2b3 codebase assessment sweep; finding adversarially verified against HEAD (`chore/release-0.8.2b3`).