feat(block-reply): deliver intermediate text during tool iterations with 2-tier config (#55)

Add block.reply event that delivers intermediate assistant text to non-streaming
channels during multi-tool iterations. Includes 2-tier config toggle:
gateway-level default (disabled) + per-channel override (inherit/on/off).

Backend:
- Emit block.reply events from agent loop between tool iterations
- Add BlockReply *bool to GatewayConfig and all 6 channel config structs
- Add BlockReplyChannel interface with ResolveBlockReply() resolution
- Guard delivery in HandleAgentEvent by RunContext.BlockReplyEnabled
- Resolve config at RegisterRun time, pass to consumer goroutine
- Conditional dedup: skip final message if identical to last block reply

UI:
- Gateway settings: Switch toggle for global default
- Per-channel: tri-state select (Inherit from gateway / Enabled / Disabled)
- Protocol: BLOCK_REPLY constant in AgentEventTypes
- Form: coerceBoolSelects for proper JSON boolean serialization

pkg/protocol/events.go

@@ -72,6 +72,7 @@ const (
 	AgentEventRunRetrying  = "run.retrying"
 	AgentEventToolCall     = "tool.call"
 	AgentEventToolResult   = "tool.result"
+	AgentEventBlockReply   = "block.reply"
 )
 
 // Chat event subtypes (in payload.type)