refactor(mcp): simplify history implementation

- Reduce history.ts from 169 to 51 lines
- Remove AI tools (list_history, restore_version, get_version)
- Remove /api/update-svg endpoint
- Remove 10-second history polling
- Simplify HistoryEntry to just {xml, svg}
- Use array index instead of version numbers

Total reduction: 1936 → 923 lines (-52%)

packages/mcp-server/src/history.ts

@@ -1,169 +1,51 @@
 /**
- * Diagram Version History for MCP Server
- *
- * Stores diagram versions in-memory per session.
- * Enables users and AI to restore previous diagram states.
+ * Simple diagram history - matches Next.js app pattern
+ * Stores {xml, svg} entries in a circular buffer
  */
 
 import { log } from "./logger.js"
 
-export interface HistoryEntry {
-    version: number
-    xml: string
-    svg: string // SVG data for thumbnail preview
-    source: "ai" | "human"
-    tool?: string // Which tool made the change (display_diagram, edit_diagram, browser_sync)
-    timestamp: Date
-    description?: string
-}
+const MAX_HISTORY = 20
+const historyStore = new Map<string, Array<{ xml: string; svg: string }>>()
 
-interface SessionHistory {
-    entries: HistoryEntry[]
-    nextVersion: number
-}
-
-// In-memory history store keyed by session ID
-const historyStore = new Map<string, SessionHistory>()
-
-// Configuration
-const MAX_HISTORY_ENTRIES = 50
-
-/**
- * Add a new entry to session history
- * Returns the assigned version number
- */
-export function addHistoryEntry(
-    sessionId: string,
-    entry: Omit<HistoryEntry, "version">,
-): number {
+export function addHistory(sessionId: string, xml: string, svg = ""): number {
     let history = historyStore.get(sessionId)
     if (!history) {
-        history = { entries: [], nextVersion: 1 }
+        history = []
         historyStore.set(sessionId, history)
     }
 
-    // Deduplicate: skip if XML is identical to last entry
-    const lastEntry = history.entries[history.entries.length - 1]
-    if (lastEntry && lastEntry.xml === entry.xml) {
-        log.debug(`Skipping duplicate history entry for session ${sessionId}`)
-        return lastEntry.version
+    // Dedupe: skip if same as last entry
+    const last = history[history.length - 1]
+    if (last?.xml === xml) {
+        return history.length - 1
     }
 
-    const version = history.nextVersion++
-    const newEntry: HistoryEntry = {
-        ...entry,
-        version,
+    history.push({ xml, svg })
+
+    // Circular buffer
+    if (history.length > MAX_HISTORY) {
+        history.shift()
     }
 
-    history.entries.push(newEntry)
-
-    // Prune oldest entries if over limit (circular buffer)
-    if (history.entries.length > MAX_HISTORY_ENTRIES) {
-        const removed = history.entries.shift()
-        log.debug(`Pruned oldest history entry v${removed?.version}`)
-    }
-
-    log.info(
-        `Added history v${version} for session ${sessionId} (source: ${entry.source}, entries: ${history.entries.length})`,
-    )
-
-    return version
+    log.debug(`History: session=${sessionId}, entries=${history.length}`)
+    return history.length - 1
 }
 
-/**
- * Get history entries for a session
- * Returns newest first, limited to specified count
- */
 export function getHistory(
     sessionId: string,
-    limit: number = 20,
-): HistoryEntry[] {
-    const history = historyStore.get(sessionId)
-    if (!history) {
-        return []
-    }
-
-    // Return newest first
-    return [...history.entries].reverse().slice(0, limit)
+): Array<{ xml: string; svg: string }> {
+    return historyStore.get(sessionId) || []
 }
 
-/**
- * Get a specific version from history
- */
-export function getVersion(
+export function getHistoryEntry(
     sessionId: string,
-    version: number,
-): HistoryEntry | undefined {
+    index: number,
+): { xml: string; svg: string } | undefined {
     const history = historyStore.get(sessionId)
-    if (!history) {
-        return undefined
-    }
-
-    return history.entries.find((e) => e.version === version)
+    return history?.[index]
 }
 
-/**
- * Get the latest version number for a session
- */
-export function getLatestVersion(sessionId: string): number {
-    const history = historyStore.get(sessionId)
-    if (!history || history.entries.length === 0) {
-        return 0
-    }
-    return history.entries[history.entries.length - 1].version
-}
-
-/**
- * Clear history for a session (used on session expiry)
- */
 export function clearHistory(sessionId: string): void {
-    if (historyStore.has(sessionId)) {
-        historyStore.delete(sessionId)
-        log.info(`Cleared history for session ${sessionId}`)
-    }
-}
-
-/**
- * Update the SVG of the latest entry (or specific version) that has empty SVG
- * Used when browser generates SVG after loading AI diagram
- */
-export function updateLatestEntrySvg(
-    sessionId: string,
-    svg: string,
-    targetVersion?: number,
-): boolean {
-    const history = historyStore.get(sessionId)
-    if (!history || history.entries.length === 0) {
-        return false
-    }
-
-    // Find entry to update - either specific version or latest without SVG
-    let entry: HistoryEntry | undefined
-    if (targetVersion !== undefined) {
-        entry = history.entries.find((e) => e.version === targetVersion)
-    } else {
-        // Find most recent entry without SVG
-        for (let i = history.entries.length - 1; i >= 0; i--) {
-            if (!history.entries[i].svg) {
-                entry = history.entries[i]
-                break
-            }
-        }
-    }
-
-    if (entry && !entry.svg) {
-        entry.svg = svg
-        log.debug(`Updated SVG for history v${entry.version}`)
-        return true
-    }
-
-    return false
-}
-
-/**
- * Get count of history entries for a session
- */
-export function getHistoryCount(sessionId: string): number {
-    const history = historyStore.get(sessionId)
-    return history?.entries.length || 0
+    historyStore.delete(sessionId)
 }