agentplane: make sampler RPC handlers owner-private - #16229

Move Sampler's gRPC implementation and its tests from the service root into internal/rpc. The command composes the concrete Store, runtime, resolver, settings and wakeup hub; public protobuf contracts remain in api/.

All twelve handlers preserve account/UID authorization, fixed status mapping, transaction-scoped admission, post-commit runtime wakeup, Follow revalidation and provider-request streaming. The request-read integration test uses the real Store and a real prepared request large enough for multiple chunks; it checks exact retained/transmitted bytes and credential redaction, plus cross-account, wrong-session and missing-request denial.

Validation: RPC unit/integration test targets and both command targets compile on RBE; formatting, Gazelle and commit hooks pass. Parent review confirms all 141 production SQL literals and all 290 existing sampler test functions remain, with every tracked sampler test file registered in a go_test. Both RPC behavioral suites pass: unit (4.2s) and real PostgreSQL integration (28.1s). Hosted CI checks the published head separately.

Proposed, ready successor to #16228 in native stack #16208. The Executor extraction is independent after Store; the final resident-runtime move updates RPC's concrete Runtime import without a forwarding package. No auth, protocol, persistence or lifecycle redesign is included.

• Test/binary compilation: https://www.buildbuddy.io/invocation/e59f4b69-1fad-43b9-b2a9-d3903becae4d
• RPC unit/integration: https://www.buildbuddy.io/invocation/315b0276-eaef-4637-8642-fe8f895c1e2a
• Review commit: b3cfd10ca9.