fix: inject correct skill path into agent context for sandbox and loc…

[niiish32x]

…al modes

• Persist skill path (project_skill_dir/) to DB on create/update when path is not explicitly provided, so legacy records without path are self-healing after next save
• Fix var_skills in ReActMasterAgent to resolve skill paths correctly: use sandbox client's skill_dir when the skill directory exists in the sandbox, otherwise fall back to the local DATA_DIR/skill directory
• Add derisk-ob.toml configuration

Description

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[yhjun1026]

LGTM