Add background agent task tools for concurrent sub-agent dispatch (#863)

[jfrey]

Summary

Implements concurrent sub-agent orchestration from #863 by adding four background agent tools that mirror the existing shell background job pattern:

• run_background_agent — dispatch a named sub-agent task asynchronously, returns a task ID immediately
• list_background_agents — show all tasks with status and runtime
• view_background_agent — inspect live buffered output or final result by task ID
• stop_background_agent — cancel a running task

Tools are registered automatically for any agent that has sub_agents configured, alongside the existing transfer_task tool.

Architecture

Handler and task tracking live in pkg/tools/builtin/agent/, following the same self-contained pattern as the shell tool in builtin/shell.go. A Runner interface abstracts the runtime dependency:

type Runner interface {
    CurrentAgentSubAgentNames() []string
    RunAgent(ctx context.Context, params RunParams) *RunResult
}

LocalRuntime implements Runner and registers the handlers via thin delegation methods, matching the existing pattern used by handleTaskTransfer, handleHandoff, etc.

Key design decisions

• Session-scoped agent identity — session.WithAgentName pins each background session to its target agent, so RunStream resolves configuration from the session rather than the shared currentAgent field. This eliminates the race between concurrent background tasks.
• Async / fire-and-forget — caller gets a task ID back immediately and polls with view_background_agent, matching the UX of shell background jobs
• ToolsApproved=true for background sessions — no user present to answer approval prompts; documented with a TODO for per-session permission scoping
• Resource caps — max 20 concurrent tasks, 100 total; completed tasks auto-pruned; live output buffer capped at 10 MB per task
• Typed enum — taskStatus int32 with iota, google/uuid for task IDs
• CAS-based status transitions for safe concurrent stop/complete

Test plan

• 28 unit tests in pkg/tools/builtin/agent/ with race detector
• go build ./..., go vet ./... clean
• Manual test: build binary and exercise background agent dispatch in a team with sub-agents

🤖 Generated with Claude Code

[dgageot]

LGTM in general! Sorry, I left a few nit-picky comments. Feel free to ignore.
One of the things that could be better is that most of the code lives in the runtime package that we wish would be simpler. Do you think most of it could be extracted?

[dgageot]

We should maybe introduce a type for the enum rather than int32?

[jfrey]

LGTM in general! Sorry, I left a few nit-picky comments. Feel free to ignore. One of the things that could be better is that most of the code lives in the runtime package that we wish would be simpler. Do you think most of it could be extracted?

I'll take care of all those and will refactor the background agent into a builtin task. It is similar to shell but has a dependency on some of the stream interfaces so I can work out an injectable interface for that to segregate the code more.