refactor(session): consolidate session and agent runtime loops

[xiami762]

Summary

• Consolidate session execution into SessionLoop, LoopContext, AgentLoop, and StepEngine.
• Centralize per-turn runtime prompt context and system-prompt assembly.
• Remove the obsolete prompt task/subtask runtime while preserving todo, delegate_task, schedule_task, and workflow behavior.
• Preserve replay-safe requests, memory bootstrap, failover, abort, and single terminal-error semantics.

Runtime architecture

SessionLoop
    -> LoopContext
    -> AgentLoop
        -> StepEngine

Compatibility

• SessionLoop remains the public entry point.
• Toolset and callable-tool selection semantics are unchanged.
• No database schema or migration changes.

Split PRs

• refactor(task): rename scheduled task manager #696 renames TaskManager to ScheduleTaskManager.
• fix(webui): render bash tool errors once #694 deduplicates WebUI bash error rendering.

Validation

• 959 session/runtime tests passed.
• 32 task tests passed.
• Ruff passed for all changed Python files.