fix: use centralized credentials from ~/.claude-mem/.env to prevent API key hijacking (#733)

This fixes Issue #733 where claude-mem would incorrectly use ANTHROPIC_API_KEY from
random project .env files instead of the user's configured Claude Code CLI subscription.

Root cause: The SDK's `query()` function inherits from `process.env` when no `env`
option is passed. When users work in projects with their own .env files containing
API keys, the SDK would discover and use those keys, billing the wrong account.

Solution: Centralized credential management via ~/.claude-mem/.env

Changes:
- Add EnvManager.ts: Centralized credential storage and isolated env builder
- SDKAgent: Pass isolated env to SDK query() that only includes credentials from
  ~/.claude-mem/.env, not random keys from process.env inheritance
- GeminiAgent/OpenRouterAgent: Use getCredential() instead of process.env fallback
- SettingsDefaultsManager: Add CLAUDE_MEM_CLAUDE_AUTH_METHOD setting ('cli' | 'api')

How it works:
1. buildIsolatedEnv() creates a clean environment with only essential system vars
   (PATH, HOME, etc.) and credentials explicitly configured in ~/.claude-mem/.env
2. SDK subprocess runs with this isolated env, never seeing random API keys
3. If no ANTHROPIC_API_KEY is in ~/.claude-mem/.env, Claude Code CLI billing is used
4. Same pattern applied to Gemini/OpenRouter agents for consistency

This ensures claude-mem always uses the user's intended billing method, regardless
of what .env files exist in their working directory.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/services/worker/SDKAgent.ts

@@ -17,6 +17,7 @@ import { logger } from '../../utils/logger.js';
 import { buildInitPrompt, buildObservationPrompt, buildSummaryPrompt, buildContinuationPrompt } from '../../sdk/prompts.js';
 import { SettingsDefaultsManager } from '../../shared/SettingsDefaultsManager.js';
 import { USER_SETTINGS_PATH, OBSERVER_SESSIONS_DIR, ensureDir } from '../../shared/paths.js';
+import { buildIsolatedEnv, getAuthMethodDescription } from '../../shared/EnvManager.js';
 import type { ActiveSession, SDKUserMessage } from '../worker-types.js';
 import { ModeManager } from '../domain/ModeManager.js';
 import { processAgentResponse, type WorkerRef } from './agents/index.js';
@@ -76,13 +77,20 @@ export class SDKAgent {
     // NEVER use contentSessionId for resume - that would inject messages into the user's transcript!
     const hasRealMemorySessionId = !!session.memorySessionId;
 
+    // Build isolated environment from ~/.claude-mem/.env
+    // This prevents Issue #733: random ANTHROPIC_API_KEY from project .env files
+    // being used instead of the configured auth method (CLI subscription or explicit API key)
+    const isolatedEnv = buildIsolatedEnv();
+    const authMethod = getAuthMethodDescription();
+
     logger.info('SDK', 'Starting SDK query', {
       sessionDbId: session.sessionDbId,
       contentSessionId: session.contentSessionId,
       memorySessionId: session.memorySessionId,
       hasRealMemorySessionId,
       resume_parameter: hasRealMemorySessionId ? session.memorySessionId : '(none - fresh start)',
-      lastPromptNumber: session.lastPromptNumber
+      lastPromptNumber: session.lastPromptNumber,
+      authMethod
     });
 
     // Debug-level alignment logs for detailed tracing
@@ -103,6 +111,7 @@ export class SDKAgent {
     // Use custom spawn to capture PIDs for zombie process cleanup (Issue #737)
     // Use dedicated cwd to isolate observer sessions from user's `claude --resume` list
     ensureDir(OBSERVER_SESSIONS_DIR);
+    // CRITICAL: Pass isolated env to prevent Issue #733 (API key pollution from project .env files)
     const queryResult = query({
       prompt: messageGenerator,
       options: {
@@ -118,7 +127,8 @@ export class SDKAgent {
         abortController: session.abortController,
         pathToClaudeCodeExecutable: claudePath,
         // Custom spawn function captures PIDs to fix zombie process accumulation
-        spawnClaudeCodeProcess: createPidCapturingSpawn(session.sessionDbId)
+        spawnClaudeCodeProcess: createPidCapturingSpawn(session.sessionDbId),
+        env: isolatedEnv  // Use isolated credentials from ~/.claude-mem/.env, not process.env
       }
     });