Fix backend stability and local sandbox regressions

[corazzione]

Fixes #1095
Fixes #1261
Fixes #1452
Fixes #1473
Fixes #1513
Fixes #1543

Summary

This PR batches several backend stability and local-sandbox fixes that were reported independently but share the same execution path.

Included fixes

• promote deferred tools after tool_search returns their schema so the tool becomes callable on the next turn
• complete the shared agent factory migration so make_lead_agent uses the same middleware/features as the reusable factory entrypoint
• move large-file conversion work off the event loop to avoid uploads/list timeouts after heavy uploads
• treat only supported Feishu slash commands as commands, so file paths beginning with / are handled as normal chat input
• allow configured sandbox.mounts paths in LocalSandbox, while still enforcing read-only mounts at the tool gate
• reduce gateway reload loops in local development by excluding sandbox output directories from hot reload watches
• reorder write_file tool arguments so path and content are the primary required fields exposed to the tool schema

Root causes

• deferred MCP tools were discovered but never promoted out of the deferred filter
• the lead-agent construction path had drifted from the shared factory runtime
• synchronous file conversion could block the event loop during large uploads
• Feishu treated any leading slash as a command marker
• LocalSandbox config supported mounts, but the local tool security layer still rejected mounted virtual paths
• gateway dev reload watched generated sandbox output paths
• write_file exposed an unused description field as the first required schema parameter

Validation

• ./.venv/Scripts/python.exe -m pytest tests/test_tool_search.py tests/test_create_deerflow_agent.py tests/test_lead_agent_model_resolution.py tests/test_file_conversion.py tests/test_feishu_parser.py tests/test_sandbox_tools_security.py -q
• ./.venv/Scripts/python.exe -m ruff check packages/harness/deerflow/tools/builtins/tool_search.py packages/harness/deerflow/agents/factory.py packages/harness/deerflow/agents/features.py packages/harness/deerflow/agents/lead_agent/agent.py packages/harness/deerflow/utils/file_conversion.py packages/harness/deerflow/sandbox/tools.py packages/harness/deerflow/sandbox/local/local_sandbox_provider.py app/channels/feishu.py tests/test_tool_search.py tests/test_create_deerflow_agent.py tests/test_lead_agent_model_resolution.py tests/test_file_conversion.py tests/test_feishu_parser.py tests/test_sandbox_tools_security.py
• cmd /c scripts\\run-with-git-bash.cmd ./scripts/test_gateway_reload_flags.sh

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