feat: add explicit branch access for memory operations

[aptend]

Summary

• add explicit branch selection for memory store/retrieve/search/list/get/correct/purge/observe paths
• route memory history and master fallback lookups through explicit branch selection
• constrain batch store to a single top-level branch; per-memory branch must be empty or match the batch branch
• keep legacy checkout behavior when branch is omitted
• tighten group-mode main write guards so branch-aware writes can target non-main safely while main-only governance remains blocked
• forward branch through MCP and remote MCP clients

Motivation

This PR supports concurrent subagent exploration. Multiple subagents using the same Memoria user can now operate on different branches by passing branch explicitly, instead of racing on the shared mem_user_state.active_branch checkout pointer.

Validation

• cargo clippy -- -D warnings
• cargo test --lib -p memoria-api
• cargo test --lib -p memoria-core -p memoria-service -p memoria-mcp
• git diff --check

Note: MatrixOne-backed branch_ops integration test was added, but local execution previously timed out because the database was unavailable.