Agents: CRUD, DB table, session binding, replace model picker

- agents table with name, avatar, description, model, persona, instructions, soft-delete
- AgentEndpoint (GET/POST/PUT/DELETE /api/agents)
- sessions + chains now reference agent_id
- ChatEndpoint resolves agent from session (no agent = no chat)
- system prompt built from agent persona + instructions
- frontend: agent editor with avatar upload, agent cards on dashboard
- session: agent dropdown replaces model picker, choice persisted

Services/DatabaseService.cs

@@ -78,10 +78,83 @@ public class DatabaseService
                 FOREIGN KEY (chain_id) REFERENCES chains(chain_id)
             );
 
+            CREATE TABLE IF NOT EXISTS agents (
+                id            INTEGER PRIMARY KEY AUTOINCREMENT,
+                agent_id      TEXT NOT NULL UNIQUE,
+                name          TEXT NOT NULL,
+                avatar        TEXT,
+                description   TEXT,
+                model         TEXT NOT NULL DEFAULT 'sonnet',
+                persona       TEXT,
+                instructions  TEXT,
+                is_deleted    INTEGER NOT NULL DEFAULT 0,
+                created_at    TEXT NOT NULL,
+                updated_at    TEXT NOT NULL
+            );
+
             CREATE INDEX IF NOT EXISTS idx_chains_session ON chains(session_id);
             CREATE INDEX IF NOT EXISTS idx_chains_archive ON chains(archive_id);
             CREATE INDEX IF NOT EXISTS idx_messages_chain ON messages(chain_id);
         ");
+
+        // Migrations
+        TryAddColumn(conn, "sessions", "agent_id", "TEXT");
+        TryAddColumn(conn, "chains", "agent_id", "TEXT");
+    }
+
+    private void TryAddColumn(SqliteConnection conn, string table, string column, string type)
+    {
+        try { conn.Execute($"ALTER TABLE {table} ADD COLUMN {column} {type}"); }
+        catch { /* column already exists */ }
+    }
+
+    // ── Agents ──
+
+    public async Task<AgentRecord> CreateAgent(AgentRecord agent)
+    {
+        using var conn = CreateConnection();
+        var now = DateTime.UtcNow.ToString("o");
+        agent.AgentId = Guid.NewGuid().ToString("N");
+        agent.CreatedAt = now;
+        agent.UpdatedAt = now;
+        await conn.ExecuteAsync(@"
+            INSERT INTO agents (agent_id, name, avatar, description, model, persona, instructions, is_deleted, created_at, updated_at)
+            VALUES (@AgentId, @Name, @Avatar, @Description, @Model, @Persona, @Instructions, 0, @CreatedAt, @UpdatedAt)
+        ", agent);
+        return agent;
+    }
+
+    public async Task<IEnumerable<AgentRecord>> GetAgents()
+    {
+        using var conn = CreateConnection();
+        return await conn.QueryAsync<AgentRecord>(
+            "SELECT * FROM agents WHERE is_deleted = 0 ORDER BY name");
+    }
+
+    public async Task<AgentRecord> GetAgent(string agentId)
+    {
+        using var conn = CreateConnection();
+        return await conn.QuerySingleOrDefaultAsync<AgentRecord>(
+            "SELECT * FROM agents WHERE agent_id = @agentId AND is_deleted = 0", new { agentId });
+    }
+
+    public async Task UpdateAgent(AgentRecord agent)
+    {
+        using var conn = CreateConnection();
+        agent.UpdatedAt = DateTime.UtcNow.ToString("o");
+        await conn.ExecuteAsync(@"
+            UPDATE agents SET name = @Name, avatar = @Avatar, description = @Description,
+                model = @Model, persona = @Persona, instructions = @Instructions, updated_at = @UpdatedAt
+            WHERE agent_id = @AgentId
+        ", agent);
+    }
+
+    public async Task DeleteAgent(string agentId)
+    {
+        using var conn = CreateConnection();
+        await conn.ExecuteAsync(
+            "UPDATE agents SET is_deleted = 1, updated_at = @now WHERE agent_id = @agentId",
+            new { agentId, now = DateTime.UtcNow.ToString("o") });
     }
 
     // ── Sessions ──
@@ -134,6 +207,14 @@ public class DatabaseService
             new { sessionId, name, now = DateTime.UtcNow.ToString("o") });
     }
 
+    public async Task UpdateSessionAgent(string sessionId, string agentId)
+    {
+        using var conn = CreateConnection();
+        await conn.ExecuteAsync(
+            "UPDATE sessions SET agent_id = @agentId, updated_at = @now WHERE session_id = @sessionId",
+            new { sessionId, agentId, now = DateTime.UtcNow.ToString("o") });
+    }
+
     // ── Archives ──
 
     public async Task<ArchiveRecord> CreateArchive(string sessionId, string name = null)
@@ -195,8 +276,8 @@ public class DatabaseService
         using var conn = CreateConnection();
         chain.CreatedAt = DateTime.UtcNow.ToString("o");
         await conn.ExecuteAsync(@"
-            INSERT INTO chains (chain_id, session_id, archive_id, model, system_prompt, input_tokens, output_tokens, iterations, stop_reason, duration_ms, created_at)
-            VALUES (@ChainId, @SessionId, @ArchiveId, @Model, @SystemPrompt, @InputTokens, @OutputTokens, @Iterations, @StopReason, @DurationMs, @CreatedAt)
+            INSERT INTO chains (chain_id, session_id, archive_id, agent_id, model, system_prompt, input_tokens, output_tokens, iterations, stop_reason, duration_ms, created_at)
+            VALUES (@ChainId, @SessionId, @ArchiveId, @AgentId, @Model, @SystemPrompt, @InputTokens, @OutputTokens, @Iterations, @StopReason, @DurationMs, @CreatedAt)
         ", chain);
     }