refactor: remove terminal title update feature

OpenCode now supports terminal title updates natively (since v1.0.150,
commit 8346550), making this plugin feature redundant. Remove the
entire terminal title feature and clean up associated dead code.

Ref: https://github.com/sst/opencode/commit/8346550

Removed:
- src/features/terminal/ (title.ts, index.ts)
- src/features/claude-code-session-state/detector.ts (dead code)
- src/features/claude-code-session-state/types.ts (dead code)
- Session title tracking (setCurrentSession, getCurrentSessionTitle)
- Terminal title update calls from event handlers

Retained:
- subagentSessions (used by background-agent, session-notification)
- mainSessionID tracking (used by session recovery)

🤖 Generated with [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/features/claude-code-session-state/state.ts

@@ -1,31 +1,11 @@
-import type { SessionErrorState, SessionInterruptState } from "./types"
-
-export const sessionErrorState = new Map<string, SessionErrorState>()
-export const sessionInterruptState = new Map<string, SessionInterruptState>()
 export const subagentSessions = new Set<string>()
-export const sessionFirstMessageProcessed = new Set<string>()
 
-export let currentSessionID: string | undefined
-export let currentSessionTitle: string | undefined
 export let mainSessionID: string | undefined
 
-export function setCurrentSession(id: string | undefined, title: string | undefined) {
-  currentSessionID = id
-  currentSessionTitle = title
-}
-
 export function setMainSession(id: string | undefined) {
   mainSessionID = id
 }
 
-export function getCurrentSessionID(): string | undefined {
-  return currentSessionID
-}
-
-export function getCurrentSessionTitle(): string | undefined {
-  return currentSessionTitle
-}
-
 export function getMainSessionID(): string | undefined {
   return mainSessionID
 }