refactor: massive codebase restructuring — naming, splitting, DRY

Crate renames:
  - zesdex-entities::seaorm → domain (misleading name, no SeaORM used)
  - zesdex-dto → merged into zesdex-entities (100% re-exports)
  - zesdex-libs → zesdex-infra (vague name)

Module renames:
  - app/harness → guard (misleading: safety gatekeeper, not test harness)
  - runtime/commands → action_dispatch (name clashed with controller/command)
  - resources → prompts (embedded prompt text, not general resources)
  - tool/seqthink → sequential_think (unreadable abbreviation)
  - msglog/query → insert (module only inserts, never queries)

Dead code removal:
  - app/mode/help.rs (orphaned — not declared in mod.rs)
  - app/mode/loading.rs (orphaned — not declared in mod.rs)

File splitting (71 new files, avg ~115 lines/file):
  - app/runtime/actions/: 1→8 files (was 2030 lines)
  - view/overlays/: 1→16 files (was 1167 lines)
  - tool/lsp/: 1→8 per-tool files (was 909 lines)
  - main.rs: 1→5 files (session, daemon, attach, event_loop)
  - workflow/engine + hive_mind: 2→10 files
  - subagent/engine + auto: 2→9 files
  - lsp/provisioner: 1→5 files
  - review/: 1→6 files
  - guard/: 1→2 files (extracted patterns)
  - state/misc: 1→3 files (input, scroll)
  - mcp/: 1→3 files (transport, adapter)
  - stream/json_repair extracted from turn.rs

DRY:
  - Pattern constants (STUB_PATTERNS etc) in guard/patterns shared with subagent
  - 3 near-identical background spawners → 1 generic + thin wrappers
  - Shared spawn_subagent_with_drain() extracted
  - Shared create_session() in main
  - write_osc52 deduplicated

Bug fixes:
  - archive_message(): sess.db → db (wrong variable name)
  - execute_one_tool(): wrong parameter name
  - check_credential_read() function was missing (restored from test expectations)

crates/zesdex-backend/src/app/subagent/spawn.rs

@@ -1,5 +1,11 @@
 //! `AgentDefinition` -- declarative specification for instantiating a
 //! subagent from workflow scripts or programmatic calls.
+//!
+//! Also provides a shared [`spawn_subagent_with_drain`] helper that
+//! eliminates the channel-creation + drain-thread boilerplate duplicated
+//! across `auto/mod.rs`, `review/mod.rs`, and `workflow/engine/mod.rs`.
+
+use super::event::SubagentEvent;
 use serde::{Deserialize, Serialize};
 
 /// Declarative specification for instantiating a subagent: name, role,
@@ -47,3 +53,51 @@ impl AgentDefinition {
     }
 
 }
+
+/// Shared subagent spawning utility: creates an mpsc channel and spawns a
+/// drain thread that forwards every [`SubagentEvent`] to `on_event`.
+///
+/// Returns the sender half (for passing to [`run_subagent`](super::engine::run_subagent))
+/// and the drain thread's join handle so the caller can keep it alive for
+/// the duration of the subagent run.
+///
+/// # Example
+///
+/// ```ignore
+/// let (tx, _drain) = spawn_subagent_with_drain(|event| {
+///     match &event {
+///         SubagentEvent::ToolCall { tool, .. } => tracing::debug!("tool: {tool}"),
+///         SubagentEvent::Completed => tracing::debug!("done"),
+///         _ => {}
+///     }
+/// });
+/// let verdict = run_subagent(&ctx, &tx)?;
+/// ```
+///
+/// # Duplication eliminated
+///
+/// Previously every subagent caller inlined the same 5-line pattern:
+///
+/// ```ignore
+/// let (tx, mut rx) = tokio::sync::mpsc::channel(32);
+/// let _drain = std::thread::spawn(move || {
+///     while let Some(event) = rx.blocking_recv() { ... }
+/// });
+/// ```
+///
+/// Callers that need a larger buffer (e.g. workflow engine uses 64) should
+/// create the channel manually instead of using this helper.
+pub fn spawn_subagent_with_drain<F>(
+    on_event: F,
+) -> (tokio::sync::mpsc::Sender<SubagentEvent>, std::thread::JoinHandle<()>)
+where
+    F: Fn(SubagentEvent) + Send + 'static,
+{
+    let (tx, mut rx) = tokio::sync::mpsc::channel(32);
+    let drain = std::thread::spawn(move || {
+        while let Some(event) = rx.blocking_recv() {
+            on_event(event);
+        }
+    });
+    (tx, drain)
+}