feat(slimrpc-spec): document hierarchical config in custom protocol binding spec

[Tehsmash]

Context

Closes part of #1786. Blocked by #1802.

The SLIMRPC custom protocol binding spec lives in a separate repository. Once the hierarchical config loading behaviour is implemented and validated in slim-bindings (#1802), the spec must be updated to formally document it so that third-party and future implementations can conform.

Proposal

Add a section to the SLIMRPC custom protocol binding spec covering:

• The config file name and search order (current dir → parent dirs → ~/.slim/config.yaml)
• The minimum required fields (node.address, identity.*)
• Environment variable override names and precedence rules
• Behaviour when no config file is found (error vs. sensible defaults)

Acceptance Criteria

• Spec section written and merged in the spec repository
• Config file schema (field names, types, defaults) fully specified
• Environment variable names and override semantics documented
• Examples included for local dev, CI, and production identity scenarios