Allow owners to read agent memories

[tlongwell-block]

Summary

• add --agent <pubkey> to read-only sprout mem ls/get/hash
• use the CLI identity as owner when --agent is provided, querying authors=[agent] and #p=[owner]
• keep mem set/patch/rm on the existing agent-side write path

Test plan

• cargo test -p sprout-cli
• cargo fmt --check
• git diff --check
• pre-push hook (full suite) passed

[tlongwell-block]

Local e2e added/ran per TESTING.md on a fresh local relay.

Setup:

• built release binaries: cargo build --release -p sprout-relay -p sprout-cli -p sprout-admin
• created a throwaway Postgres DB: sprout_owner_mem_e2e_1779482074_17329
• applied schema/schema.sql via pgschema
• started target/release/sprout-relay on isolated ports 3030/8088/9202 with an isolated Typesense collection

Flow verified:

1. generated owner + agent keypairs
2. as agent: sprout mem set core ... --owner <owner>
3. as agent: sprout mem set e2e-owner-read ... --owner <owner>
4. as owner: sprout mem get core --agent <agent> returned exact core value
5. as owner: sprout mem get e2e-owner-read --agent <agent> returned exact memory value
6. as owner: sprout mem hash core --agent <agent> matched local SHA-256
7. as owner: sprout mem hash e2e-owner-read --agent <agent> matched local SHA-256
8. as owner: sprout mem ls --agent <agent> --json listed mem/e2e-owner-read and did not list core

Relay confirmed two kind:30174 ingests for the agent-authored engrams. E2E PASS.