docs(gateway): document progress cleanup setting

[helix4u]

What does this PR do?

Documents the gateway display.cleanup_progress setting added by bf843adf0 / #21186. The setting keeps live progress visible during a turn and then deletes temporary progress/status bubbles after successful final delivery on platforms that support deletion.

Related Issue

Follow-up to docs drift from bf843adf0 / #21186.

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• Added display.cleanup_progress to the display settings example.
• Documented global and per-platform cleanup configuration.
• Explained current behavior: best-effort deletion on adapters that support it, primarily Telegram, and failed runs leave progress breadcrumbs in place.

How to Test

1. Review the docs against gateway/display_config.py and the cleanup path in gateway/run.py.
2. Confirm git diff --check passes.
3. Docs-only change; no runtime tests run.

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
• I’ve added tests for my changes (required for bug fixes, strongly encouraged for features)
• I’ve tested on my platform: N/A docs-only

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
• 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

For New Skills

• This skill is broadly useful to most users (if bundled) — see Contributing Guide
• SKILL.md follows the standard format (frontmatter, trigger conditions, steps, pitfalls)
• No external dependencies that aren’t already available (prefer stdlib, curl, existing Hermes tools)
• I’ve tested the skill end-to-end: hermes --toolsets skills -q "Use the X skill to do Y"

Screenshots / Logs

git diff --check passed.

[pefontana]

cleanup_progress already got documented on main via #28497 (messaging/index.md) plus cli-config.yaml.example, so this one's covered now.