refactor: move 15 benchmark DomainConfigs out of agent/domain.py to configs (CODE_HEALTH #3)

[duanyiqun]

CODE_HEALTH #3 (see docs/CODE_HEALTH.md in #37).

src/cognifold/agent/domain.py (1,500 lines) defines 21 module-level DomainConfig literals — 15 of them benchmark-only (LOCOMO, FUTUREX, LONGMEMEVAL, MSC, BABILONG, MUTUAL, MUSIQUE, STREAMINGQA, RGB, TIMEQA, NARRATIVEQA, QMSUM, SOCIALIQA, TOMI, SAFETYBENCH; lines 419–1310), several for benchmarks whose code was removed from the repo. The extension point register_domain() (line 1494) exists and is unused for these. The list has already drifted out of sync with query/agent.py's benchmark dispatch (TIMEQA has a domain but no query handler).

Fix direction:

• Keep the ~6 real product domains in-tree
• Move benchmark domains to YAML under configs/ (which already hosts the per-benchmark prompt profiles)
• Each runner registers its domain via register_domain() at startup
• Delete domains for removed benchmarks outright

Pairs naturally with #39 (query/agent.py de-benchmarking).