[memory→tinymemory] tinymemory-api: stable error codes, account-wide errors, typed recall skip reason, EngineStatus #1

Moved from tinyhumansai#280, which is closed in favour of this issue. Part of tinyhumansai/openhuman#8000.

For openhuman's memory/error.rs (153) and memory/status.rs (49).

• Error::code() returns the seven stable codes OpenHuman maps to RPC (MEMORY_OFF, UNSUPPORTED, INVALID_REQUEST, …). Add the Off (with reason) and InsufficientCredits kinds and Error::is_account_wide(). The account-wide check currently lives three times: host MemoryError::is_account_wide, layout_migration::error::is_account_wide, and integrations::cortex::is_insufficient_credits. Keep one.
• Bug fix: holistic_recall reports skipped sections as strings. The host recovers a typed refusal by matching Display prefixes ("unauthorized: ", "unavailable: ", "engine error: "; memory/error.rs refusal_from_skip_reason), so any wording change upstream silently turns an account-wide refusal into a skip. Return a typed skip reason instead.
• engine::EngineStatus { health, reason, capabilities, contract_version }, built from EngineDescriptor + EngineHealth, with an Off case, plus the operation list currently hard-coded in status.rs.
• Housekeeping left over from [openhuman#7523] tinymemory: move generic memory mechanics out of openhuman-core tinyhumansai/tinymemory#266: AGENTS.md/README still say "exactly three crates" (tinyredact exists) and mention tinycortex/tinymemory-import.

The host keeps only impl From<Error> for StructuredRpcError and the SubsystemStatus wire struct (senamakel/openhuman#86).

Done when: the new API is merged with its tests (moved host tests travel with the code), cargo test --workspace and cargo clippy --workspace --all-targets -- -D warnings pass, and a tag is cut so OpenHuman can bump the gitlink. tinymemory-tools stays tokio-free (spawn/clock are traits); schemars only behind an optional schema feature.

Agent brief

• Files to consider: host memory/error.rs, memory/status.rs; upstream tinymemory-api/src/{error,engine}/
• Research: the gaps listed above
• Workflow: an implementing-an-epic step; follow the migrate-to-submodule skill (upstream PR → tag → OpenHuman gitlink bump + delete host copy).
• Subagent: tdd-implementer.

Parent: tinyhumansai/openhuman#8000