Add built-in download tool

[ZhiXiao-Lin]

Summary

• add a local-workspace download tool inspired by Surge's range probe, adaptive chunking, bounded retry, sequential fallback, and atomic finalization
• share the SSRF-safe HTTP transport with web_fetch, including DNS pinning, manual redirect validation, proxy support, credential isolation, and redacted source anchors
• enforce binary-safe streaming, size/time limits, coherent validators, strict Content-Range checks, optional SHA-256, cancellation cleanup, and workspace/symlink boundaries
• wire download into permissions, HITL risk assessment, selection, queues, subagents, task projection, skill validation, and local-backend registration
• document the tool and add unit plus HTTP integration coverage

Validation

• cargo fmt --all -- --check
• cargo check -p a3s-code-core
• cargo clippy -p a3s-code-core --lib -- -D warnings
• download/governance/shared HTTP focused tests: passed
• cargo test -p a3s-code-core --test test_workspace_backend custom_workspace_file_tools_do_not_touch_local_filesystem: passed
• full lib suite: 2418 passed, 9 ignored, 1 existing Windows-only failure. agent_api::tests::test_verify_commands_builds_report_from_bash_results runs the Unix command printf ok through the Windows PowerShell-backed BashTool and fails independently when rerun in isolation.

Reference

Implementation principles were checked against Surge's probe, concurrent worker, scheduler fallback, and sync-then-rename finalization behavior.