refactor: migrate monolithic crate to Cargo Workspace with Clean Architecture

Transform the single binary crate into a 9-crate workspace monorepo:

- Root Cargo.toml as [workspace] manager with resolver = "2"
- zesdex-entities: Domain entity types (session, settings, store, message, etc.)
- zesdex-utils: Pure utility functions (error, logger, pagination, slug, clipboard)
- zesdex-dto: Data Transfer Objects for LLM provider API communication
- zesdex-ipc: Unix-socket IPC layer (client/server/framing/protocol)
- zesdex-iam: Identity & Access Management (Clean Architecture: domain/application/infrastructure)
- zesdex-cms: Content Management (Clean Architecture: domain/application/infrastructure)
- zesdex-middleware: HTTP middleware (Auth, CORS, Rate Limiting)
- zesdex-libs: Composition root (AppContext, DB init, JWT, Argon2)
- zesdex-backend: Main binary entry point + seed/migrate binaries
- DevOps: Dockerfile, docker-compose, Nix (flake/shell/default), CI/CD updates
- Remove dead root src/ and src-misc/ directories

All crate re-exports maintain backward compatibility with original
crate::model::*, crate::dto::*, crate::ipc::* module paths.
Feature crates enforce strict layer separation: domain -> application
-> infrastructure with generic trait-based dependency injection.

crates/zesdex-backend/src/model/agent_def/builtin.rs

@@ -0,0 +1,92 @@
+#![allow(dead_code)]
+//! Hardcoded built-in subagent definitions (coder, reviewer, researcher, planner).
+use crate::app::subagent::spawn::AgentDefinition;
+
+/// Build the fixed list of built-in agent definitions shipped with zesdex.
+///
+/// Flow: construct each `AgentDefinition` with a name, system prompt, and
+/// allowed tool list, then collect into a `Vec`.
+///
+/// Why: these agents are always available regardless of global/session
+/// config, giving users a baseline set of roles out of the box.
+///
+/// Return: a freshly-built `Vec<AgentDefinition>` (coder, reviewer,
+/// researcher, planner).
+pub fn builtin_agents() -> Vec<AgentDefinition> {
+    vec![
+        AgentDefinition::new(
+            "coder".to_string(),
+            "coder".to_string(),
+        ).with_system_prompt(
+            "You are a coding agent. Write correct, idiomatic Rust code.".to_string()
+        ).with_allowed_tools(
+            vec![
+                "read".to_string(),
+                "write".to_string(),
+                "edit".to_string(),
+                "bash".to_string(),
+                "grep".to_string(),
+                "glob".to_string(),
+                "git_operator".to_string(),
+                "lsp_connect".to_string(),
+                "lsp_diagnostics".to_string(),
+                "lsp_hover".to_string(),
+                "lsp_definition".to_string(),
+                "lsp_references".to_string(),
+                "lsp_completion".to_string(),
+                "lsp_disconnect".to_string(),
+            ]
+        ).with_max_steps(usize::MAX),
+
+        AgentDefinition::new(
+            "reviewer".to_string(),
+            "reviewer".to_string(),
+        ).with_system_prompt(
+            "You are a code reviewer. Focus on correctness, safety, and performance.".to_string()
+        ).with_allowed_tools(
+            vec![
+                "read".to_string(),
+                "grep".to_string(),
+                "glob".to_string(),
+                "recall".to_string(),
+                "remember".to_string(),
+                "lsp_diagnostics".to_string(),
+                "lsp_hover".to_string(),
+                "lsp_definition".to_string(),
+                "lsp_references".to_string(),
+            ]
+        ).with_max_steps(usize::MAX),
+
+        AgentDefinition::new(
+            "researcher".to_string(),
+            "researcher".to_string(),
+        ).with_system_prompt(
+            "You are a research agent. Search for information and summarize findings.".to_string()
+        ).with_allowed_tools(
+            vec![
+                "read".to_string(),
+                "grep".to_string(),
+                "glob".to_string(),
+                "bash".to_string(),
+                "search_web".to_string(),
+                "fetch_url".to_string(),
+            ]
+        ).with_max_steps(usize::MAX),
+
+        AgentDefinition::new(
+            "planner".to_string(),
+            "planner".to_string(),
+        ).with_system_prompt(
+            "You are a planning agent. Break down tasks into clear steps.".to_string()
+        ).with_allowed_tools(
+            vec![
+                "read".to_string(),
+                "write".to_string(),
+                "edit".to_string(),
+                "bash".to_string(),
+                "todo_write".to_string(),
+                "todo_finish".to_string(),
+            ]
+        ).with_max_steps(usize::MAX),
+    ]
+}