Add debug-load performance hook

[gadflysu]

Goal

Add a hidden/advanced --debug-load mode that starts aps, runs the selected load lifecycle, and exits without rendering list output, starting the TUI, or launching an agent.

This gives external benchmark tools such as hyperfine stable process boundaries for total startup/load measurement.

Dependency

Blocked by #61. Do not assume a specific load function or package shape in this issue; inspect the load lifecycle code after #61 lands.

Expected behavior

• Parse normal aps flags.
• Apply normal source selection, path filter, date filter, and cache behavior.
• Support --debug-load=complete for the complete/list-style load path.
• Support --debug-load=stream for the streaming/interactive-style load path, drained to completion without starting Bubble Tea.
• Exit 0 after the selected load path reaches its all-loaded boundary.
• Exit non-zero on fatal load/config errors.
• Do not render list rows, start Bubble Tea, start watcher/PID-cache GC for interactive UI, or launch an agent.
• Keep default output quiet so benchmark timing is not polluted by per-session output.

Non-goals

• Do not add aps perf ... subcommands.
• Do not add fixture generation.
• Do not add JSON metrics/reporting output.
• Do not add APS_HOME, --home, or --claude-projects-dir.
• Do not change normal interactive, list, or launch behavior.

Plan

See docs/agent/plan-issue-59-debug-load-performance-hook.md.