fix(runtime): guard nested agent delegation

[Sayt-0]

Summary

• reject direct and indirect nested agent-delegation cycles with the attempted path
• cap transfer_task and run_background_agent chains at 10 delegation edges
• preserve valid multi-level foreground and background delegation with session-aware caller resolution
• keep pinned background sessions isolated from the shared foreground agent
• document the delegation limit and add deterministic async end-to-end coverage

Context

This follows #3886, which establishes that agents started with run_background_agent may delegate to their own sub-agents. Supporting that pattern requires both termination guards and session-aware routing: a pinned background session must validate, attribute, and execute nested work as its pinned agent rather than the runtime's shared foreground agent.

Issue expectations

Expectation	Implementation
Direct cycles are rejected	validateDelegation rejects self-delegation before a child session starts
Indirect cycles are rejected	Per-session lineage detects an ancestor target and reports paths such as a -> b -> a
Acyclic multi-level delegation remains supported	Lineage propagates through foreground, background, and mixed nested paths
Excessive nesting stops with an actionable error	A fixed runtime maximum of 10 edges is documented; depth 10 is accepted and depth 11 is rejected
Foreground and background paths are covered	Tests cover transfer_task, RunAgent, pinned sessions, concurrent nested transfers, and real async background handlers
Background delegation from #3886 remains safe	Handler target validation resolves from the calling session; pinned children do not mutate shared foreground state

Test coverage

• direct and indirect cycles
• exact maximum-depth boundary
• foreground transfer switching and restoration
• nested transfers from pinned background sessions
• concurrent pinned nested transfers
• nested background-agent hook attribution
• forked skills from pinned sessions without adding a delegation edge
• legacy Runner compatibility fallback
• end-to-end root -> run_background_agent(worker) -> run_background_agent(helper) -> list/view through the real dispatcher and detached goroutines

Validation

• task test
• task lint
• task build
• focused runtime tests with -race, including repeated async end-to-end runs

Closes #3904
Related to #3886