feat: add external memory context APIs

[Super-Sky]

Refs #10

Completed

• Scoped app-owned memory write/query with app_id + owner_id + scope isolation.
• Context asset resolve/assemble, safe operation traces, compression summary, OpenAPI, bilingual docs, and tests.

Boundary

• Athena stores summaries only; no fund, holding, or trade business objects.
• The default in-process store is local/demo only. Durable persistence and host-principal binding remain follow-up work.

Verification

• Focused Go tests, race tests, benchmark, go vet, and absolute-path check passed.