feat(registry): add Exa MCP server authentication setup

docs/features/007-exa-authentication.md

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+# Feature #007: Exa MCP Server Authentication Setup
+
+## Overview
+
+Add authentication setup for Exa MCP server to enable AI-powered semantic search and deep research capabilities.
+
+## Implementation Plan
+
+### Phase 1: Registry Updates
+- [ ] Update `registry/mcp-servers/exa/index.ts`
+- [ ] Add `configure()` lifecycle method
+- [ ] Implement API key prompt and validation
+
+### Phase 2: Authentication Flow
+- [ ] Add interactive prompt for Exa API key
+- [ ] Validate key format (starts with `exa_`)
+- [ ] Store key in `.env.mcp.secrets` as `EXA_API_KEY`
+- [ ] Update MCP config to use environment variable
+
+### Phase 3: Documentation
+- [ ] Update `registry/mcp-servers/exa/claude.md`
+- [ ] Add authentication setup instructions
+- [ ] Document Exa capabilities and use cases
+- [ ] Add troubleshooting guide for auth issues
+
+### Phase 4: CLAUDE.md Integration
+- [ ] Ensure CLAUDE.md gets updated during init
+- [ ] Include Exa usage examples
+- [ ] Document when to use Exa vs other research tools
+
+## Technical Approach
+
+```typescript
+export const exaServer: MCPServer = {
+  name: "exa",
+  displayName: "Exa Research",
+  description: "AI-powered semantic search and deep research",
+
+  async configure(ctx: SetupContext): Promise<void> {
+    const apiKey = await ctx.prompt({
+      type: "input",
+      name: "EXA_API_KEY",
+      message: "Enter your Exa API key (get one at https://exa.ai):",
+      validate: (value: string) => {
+        if (!value.startsWith("exa_")) {
+          return "Exa API keys should start with 'exa_'";
+        }
+        return true;
+      }
+    });
+
+    await ctx.saveSecret("EXA_API_KEY", apiKey);
+  },
+
+  install(ctx: SetupContext): MCPConfig {
+    return {
+      type: "stdio",
+      command: "npx",
+      args: [
+        "-y",
+        "dotenv-cli",
+        "-e",
+        ".env.mcp.secrets",
+        "--",
+        "npx",
+        "-y",
+        "@modelcontextprotocol/server-exa"
+      ],
+      env: {}
+    };
+  }
+};
+```
+
+## Testing Plan
+
+- Test with valid Exa API key
+- Test with invalid key format
+- Test with missing API key
+- Verify deep research capabilities work
+- Test web search and crawling features
+- Verify CLAUDE.md instructions are clear
+
+## Areas for Feedback
+
+1. Should we test the API key during setup (requires network call)?
+2. How should we handle API rate limits or quota exceeded?
+3. Should we document cost implications of deep research?
+4. What example queries should we include in documentation?
+
+## Related Issues
+
+- Issue #7: https://github.com/cedric/spantree-fluent/fluent-toolkit/issues/7
+
+## References
+
+- Exa Website: https://exa.ai
+- Exa API Docs: https://docs.exa.ai
+- MCP Exa Server: https://github.com/modelcontextprotocol/servers/tree/main/src/exa
+
+## Exa Capabilities to Document
+
+### Deep Research
+- AI-powered comprehensive research with sources
+- Models: `exa-research` (fast) and `exa-research-pro` (comprehensive)
+- Poll-based workflow with `deep_researcher_start` and `deep_researcher_check`
+
+### Semantic Search
+- `web_search_exa`: Intent-aware web search
+- `company_research_exa`: Business intelligence
+- `linkedin_search_exa`: Professional profiles
+- `get_code_context_exa`: Technical documentation and code examples
+
+### Content Extraction
+- `crawling_exa`: URL-based content extraction with semantic processing