feat: agent registry API + heartbeat checker + core unit tests

Tasks completed:
- 2.7: Core unit tests (14 tests: state machine, event store, queue, timeout, retry)
- 3.1: POST /api/v1/agents/register (upsert on duplicate)
- 3.2: POST /api/v1/agents/heartbeat
- 3.3: POST /api/v1/agents/deregister (offline + requeue running tasks)
- 3.4: GET /api/v1/agents (filter by capability + status)
- 3.5: Background heartbeat checker (marks offline, sets tasks agent_lost)
- 3.6: API unit tests (register, duplicate, heartbeat, deregister, checker)

All 14 tests pass. cargo check clean (warnings only).

src/core/models.rs

@@ -1,5 +1,6 @@
 use chrono::{DateTime, Utc};
 use serde::{Deserialize, Serialize};
+use std::collections::HashMap;
 
 // ─── Agent ───────────────────────────────────────────────────────
 
@@ -15,6 +16,32 @@ pub enum AgentType {
     Other(String),
 }
 
+impl AgentType {
+    pub fn as_str(&self) -> &str {
+        match self {
+            Self::OpenClaw => "openclaw",
+            Self::ClaudeCode => "claude-code",
+            Self::CodexCli => "codex-cli",
+            Self::Hermes => "hermes",
+            Self::Acp => "acp",
+            Self::Shell => "shell",
+            Self::Other(value) => value.as_str(),
+        }
+    }
+
+    pub fn from_str(value: &str) -> Self {
+        match value {
+            "openclaw" => Self::OpenClaw,
+            "claude-code" => Self::ClaudeCode,
+            "codex-cli" => Self::CodexCli,
+            "hermes" => Self::Hermes,
+            "acp" => Self::Acp,
+            "shell" => Self::Shell,
+            other => Self::Other(other.to_string()),
+        }
+    }
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
 #[serde(rename_all = "lowercase")]
 pub enum AgentStatus {
@@ -23,7 +50,26 @@ pub enum AgentStatus {
     Draining,
 }
 
-#[derive(Debug, Clone, Serialize, Deserialize)]
+impl AgentStatus {
+    pub fn as_str(&self) -> &'static str {
+        match self {
+            Self::Online => "online",
+            Self::Offline => "offline",
+            Self::Draining => "draining",
+        }
+    }
+
+    pub fn from_str(value: &str) -> Self {
+        match value {
+            "online" => Self::Online,
+            "offline" => Self::Offline,
+            "draining" => Self::Draining,
+            _ => Self::Offline,
+        }
+    }
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
 pub struct Agent {
     pub agent_id: String,
     pub agent_type: AgentType,
@@ -34,7 +80,7 @@ pub struct Agent {
     pub status: AgentStatus,
     pub last_heartbeat_at: DateTime<Utc>,
     pub registered_at: DateTime<Utc>,
-    pub metadata: std::collections::HashMap<String, String>,
+    pub metadata: HashMap<String, String>,
 }
 
 // ─── Task ────────────────────────────────────────────────────────
@@ -75,8 +121,6 @@ pub enum Priority {
 }
 
 impl Priority {
-    /// Explicit priority ordering (lower = higher priority).
-    /// Not reliant on variant declaration order.
     pub fn order(&self) -> u8 {
         match self {
             Self::Urgent => 0,
@@ -86,7 +130,6 @@ impl Priority {
         }
     }
 
-    /// Serialize to the string stored in the DB.
     pub fn as_str(&self) -> &'static str {
         match self {
             Self::Low => "low",
@@ -97,15 +140,15 @@ impl Priority {
     }
 }
 
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
 pub struct Task {
     pub task_id: String,
-    pub source: String,       // "forgejo:<repo>#<issue>"
-    pub task_type: String,    // "code", "review", "test", "deploy", "research"
+    pub source: String,
+    pub task_type: String,
     pub priority: Priority,
     pub status: TaskStatus,
     pub assigned_agent_id: Option<String>,
-    pub requirements: String, // Issue body
+    pub requirements: String,
     pub labels: Vec<String>,
     pub created_at: DateTime<Utc>,
     pub assigned_at: Option<DateTime<Utc>>,
@@ -136,7 +179,7 @@ pub enum ArtifactType {
     Url,
 }
 
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
 pub struct Artifact {
     pub artifact_type: ArtifactType,
     pub url: Option<String>,
@@ -144,7 +187,7 @@ pub struct Artifact {
     pub description: Option<String>,
 }
 
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
 pub struct Receipt {
     pub task_id: String,
     pub agent_id: String,
@@ -157,7 +200,7 @@ pub struct Receipt {
 
 // ─── TaskEvent (event sourcing) ──────────────────────────────────
 
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 pub struct TaskEvent {
     pub event_id: String,
     pub task_id: String,