feat(tui): add Tasks panel with PanelProvider registry architecture

[yishuiliunian]

Summary

• Add a Tasks panel to the TUI panel zone, displaying TaskCreate/TaskUpdate progress alongside Agents and BgTasks panels
• Introduce PanelProvider registry architecture — unified trait + registry replaces per-kind dispatch in navigation, rendering, and sync
• Implement session state cleanup to prevent unbounded memory growth in long-running sessions (auto-evicts finished agents, completed bg_tasks)

Changes

Event pipeline (new)

• loopal-protocol: TaskSnapshot type + TasksChanged event variant
• loopal-agent: TaskStore subscribe/notify for change detection (pure persistence, no protocol dep)
• loopal-agent-server: TaskBridge converts store changes → TasksChanged events via frontend
• loopal-session: task_state.rs handles TasksChanged → SessionState.task_snapshots

Panel architecture (refactored)

• PanelProvider trait (kind, max_visible, item_ids, height, render) replaces old PanelSection trait
• PanelRegistry holds registered providers; panel_ops and render iterate over it
• providers/ directory: AgentPanelProvider, TasksPanelProvider, BgTasksPanelProvider
• PanelSectionState unifies focus/scroll per panel (replaces scattered focused_agent, focused_bg_task, etc.)
• Adding a new panel type = 1 provider file + 1 register call + 1 PanelKind variant (3 files, zero changes to panel_ops/render/tui_loop)

Session cleanup (new)

• session_cleanup.rs: removes completed bg_tasks from session state each frame
• Finished sub-agents (with persisted session_id) removed from state.agents
• bg_task_details uses merge-and-retain pattern (capped at 50) so log viewer still works
• BgTasks panel now shows Running tasks only (completed hidden immediately)

Other fixes

• Removed dead viewing indicator (▶) from agent panel (was unreachable)
• BgTask/TasksChanged events now return early in agent_handler (skip post_event_cleanup)
• Task subjects sanitized (newlines replaced with spaces)
• Scroll offset clamped each frame to prevent stale offset after items disappear

Test plan

• bazel build //... compiles
• bazel build //... --config=clippy zero warnings
• bazel test //... — 52/52 pass (includes new task_state, tasks_panel, enter_panel, panel_tab tests)