feat: add is_subagent flag to agents, fix all lint/type/test issues

- Add is_subagent checkbox to agent editor; subagents are hidden from Chat Settings
- Add is_subagent field to AgentDefinition type, TOML serialization, and API route
- Filter subagents from ModelSelector agent list
- Fix all biome lint/format errors across codebase (useLiteralKeys, noNonNullAssertion, noExplicitAny, formatting, import sorting)
- Fix svelte-check errors (type narrowing in SkillsBrowser, ToolPermissions, SidebarPanel)
- Fix a11y warnings in App.svelte (label-control associations)
- Fix test mocks missing BackgroundShellStore, BackgroundTranscriptStore, createWebSearchTool, createYoutubeTranscribeTool
- Update stale 409 test to match current message-queuing behavior
- Exclude packaging/ and release/ dirs from biome to avoid linting stale build artifacts

1 files changed, 5 insertions, 0 deletions

diff --git a/packages/core/src/agents/loader.ts b/packages/core/src/agents/loader.ts
index ec29983..cf84381 100644
--- a/packages/core/src/agents/loader.ts
+++ b/packages/core/src/agents/loader.ts
@@ -112,6 +112,10 @@ export function saveAgent(agent: AgentDefinition): void {
 		tomlContent.cwd = agent.cwd;
 	}
 
+	if (agent.is_subagent) {
+		tomlContent.is_subagent = true;
+	}
+
 	// smol-toml handles [[models]] array-of-tables
 	if (agent.models.length > 0) {
 		tomlContent.models = agent.models.map((m) => ({
@@ -202,6 +206,7 @@ function loadAgentsFromDir(dir: string, scope: string): AgentDefinition[] {
 				scope,
 				slug,
 				...(typeof parsed.cwd === "string" && parsed.cwd ? { cwd: parsed.cwd } : {}),
+				...(parsed.is_subagent === true ? { is_subagent: true } : {}),
 			});
 		} catch {
 			// Skip unparseable files