refactor: remove Anthropic API integration, CLI agent only (ADR-0028)

Remove AIChatPanel, useAIChat hook, Rust ai_chat command, and
anthropic_key from settings. AI is now exclusively via Claude CLI
subprocess (AiPanel). Simplifies codebase and eliminates API key
management for users. ADR-0027 superseded.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/adr/0027-dual-ai-architecture.md

@@ -2,7 +2,8 @@
 type: ADR
 id: "0027"
 title: "Dual AI architecture (API chat + CLI agent)"
-status: active
+status: superseded
+superseded_by: "0028"
 date: 2026-03-01
 ---
 

docs/adr/0028-cli-agent-only-no-api-key.md

@@ -0,0 +1,40 @@
+---
+type: ADR
+id: "0028"
+title: "CLI agent only — no direct Anthropic API key"
+status: active
+date: 2026-03-29
+supersedes: "0027"
+---
+
+## Context
+
+ADR-0027 introduced a dual AI architecture: a lightweight API-based chat (AIChatPanel) using the Anthropic API directly, and a full CLI agent (AiPanel) spawning Claude CLI as a subprocess with MCP tool access. In practice, the API chat was never shipped to users — the CLI agent covered all use cases and provided a superior experience through tool access and MCP integration. Maintaining two codepaths added complexity, and requiring users to manage an Anthropic API key created friction.
+
+## Decision
+
+**Remove the direct Anthropic API integration entirely. AI is available exclusively via CLI agent subprocesses (Claude Code, and in the future Codex or other CLI agents).** No API key field in settings. The CLI agent authenticates via its own mechanism (e.g. `claude` CLI login).
+
+Removed:
+- `AIChatPanel` component, `useAIChat` hook
+- Rust `ai_chat` command and `ai_chat.rs` module
+- `anthropic_key` field from Settings (Rust and TypeScript)
+- Vite dev-server Anthropic API proxy (`aiChatProxyPlugin`, `aiAgentProxyPlugin`)
+
+Kept:
+- `AiPanel` + `useAiAgent` — Claude CLI subprocess with MCP vault integration
+- Shared utilities in `ai-chat.ts` (`trimHistory`, `formatMessageWithHistory`, `streamClaudeChat`, etc.)
+- `Cmd+I` keyboard shortcut and menu item for toggling the AI panel
+
+## Options considered
+
+- **Option A** (chosen): Remove API chat, keep CLI agent only. Simplifies codebase, removes API key management, single codepath.
+- **Option B**: Keep both but hide API chat behind feature flag. Adds dead code weight without benefit.
+- **Option C**: Replace CLI agent with API chat + manual tool calling. Loses MCP integration and Claude CLI features.
+
+## Consequences
+
+- Users no longer need to obtain or manage an Anthropic API key
+- Existing saved API keys are silently ignored (the field no longer exists in the Settings struct; serde skips unknown fields on deserialization)
+- Future CLI agents (Codex, etc.) can plug into the same `AiPanel` architecture
+- If a lightweight chat mode is needed later, it should be built as a CLI agent mode, not a separate API integration