RFD: Add flag config option type for boolean toggles

[fscarponi]

This RFD proposes adding a new flag type to session configuration options, enabling agents to expose simple boolean ON/OFF toggles as first-class config options alongside the existing select type.

Motivation

Currently, the only way to expose a boolean toggle (e.g., "Brave Mode", "Read Only", "Produce Report") is to use a select with two artificial options like "on"/"off". This forces clients to implement custom, non-agnostic logic to detect which selects are actually boolean flags — defeating the purpose of a standardized protocol.

A dedicated flag type allows clients to natively render toggle switches or checkboxes without guessing.

Proposed changes

• SessionConfigFlag struct with current_value: bool
• Flag variant in SessionConfigKind (discriminated by "type": "flag")
• SessionConfigOptionValue untagged enum (String | Bool) for SetSessionConfigOptionRequest.value
• Wire-level backward compatible: existing JSON payloads with string values remain valid

Working implementation

A reference implementation is available on the branch fabrizio.scarponi/flag-config-option, including updated docs, schema regeneration, and passing tests/clippy.

Happy to open a follow-up implementation PR once this RFD is reviewed and championed