docs(delegate): drop the nonexistent delegate_task(background=true) from docs

[liuhao1024]

What does this PR do?

Three texts told agents to call delegate_task(background=true), but DELEGATE_TASK_SCHEMA (tools/delegate_tool.py) does not expose a background property — the code comment right at the schema marks it "Unadvertised; do not re-add", and _model_background_value states that top-level delegations always run in the background and the model does not choose. An agent taking the docs literally either passes an unknown key or invents a polling loop to compensate.

This PR rewrites the three spots to state the real contract on current main: a top-level delegate_task always runs detached, returns a handle immediately, and the result re-enters the conversation as a new message; an orchestrator child (depth > 0) is the exception that waits inline for its workers. Docs must describe the behavior that ships, so the wording intentionally avoids predicting whether that contract will ever change.

Related Issue

Fixes #109730

Type of Change

• 📝 Documentation update

Changes Made

• skills/autonomous-ai-agents/hermes-agent/references/background-systems.md — replaced the "Background: delegate_task(background=true)" bullet with an "Always background" bullet: top-level calls always return a handle, there is no background argument, orchestrator children wait inline.
• website/docs/user-guide/features/delegation.md — "Stall Detection" intro now says every top-level delegate_task runs detached automatically instead of showing a background=true call.
• tools/AGENTS.md — same root cause, third occurrence found while fixing the two above: the delegation section said "the parent waits for the summary unless background=true", which inverts the actual default. Rewritten to the same real contract.

How to Test

1. git grep -n "delegate_task(background" \ — should return no hits after this change (Observed result: zero hits across skills/, website/, tools/AGENTS.md, docs/).
2. sed -n '598,665p' tools/delegate_tool.py — DELEGATE_TASK_SCHEMA parameters are tasks / action / subagent_id / message only; the adjacent comment reads "background (bool) is also accepted — DEPRECATED, ignored: top-level delegations always run in the background. Unadvertised; do not re-add." The docs now match this.
3. Sanity: .venv/bin/pytest tests/tools/test_delegate.py -q — 80 passed; the 1 failure (test_child_dedicated_db_follows_parents_db_path) reproduces identically on a pristine upstream/main checkout (commit b6b53c6) in this environment, i.e. pre-existing and unrelated to a docs-only change. Observed result: docs-only diff, no code path touched.

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass (docs-only change; ran the delegation module tests/tools/test_delegate.py as sanity — 80 passed, 1 pre-existing failure reproduced identically on pristine upstream/main, see How to Test)
• I've added tests for my changes (required for bug fixes, strongly encouraged for features) — N/A, documentation-only
• I've tested on my platform: macOS (darwin 25.4.0, arm64)

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A (wording fix only, no workflow change)
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A (no behavior change)