fix: wire bump_use() into skill loading paths

[Giggitycountless]

Summary

bump_use() in tools/skill_usage.py existed and was tested, but had zero production call sites — use_count stayed at 0 and last_used_at stayed None forever, making the Curator's stale timer unable to distinguish actively-used skills from never-used ones.

Changes

Adds bump_use() calls to two skill loading paths in agent/skill_commands.py:

1. build_skill_invocation_message() — when a /skill slash command is resolved and its content is injected into the prompt
2. build_preloaded_skills_prompt() — when skills are loaded via the CLI --skills flag at session start

Both calls follow the same pattern as the existing bump_view() call in skills_tool.py: wrapped in try/except Exception for best-effort operation that never breaks skill loading.

Why these paths

Per the issue's analysis, option 1 (slash command resolution) is the most precise: a skill is "used" when its content is actually injected to solve a task, not just when it's listed or browsed. The preload path is included because it represents intentional, active use of a skill for a session.

Before

$ grep -rn 'bump_use(' tools/ --include='*.py'
tools/skill_usage.py:277:def bump_use(skill_name: str) -> None:
# ← zero production call sites

# .usage.json: all skills show use_count: 0, last_used_at: null

After

# After invoking /my-skill or loading via --skills:
# .usage.json shows use_count: 1, last_used_at: <timestamp>

Fixes #17782

[alt-glitch]

Likely duplicate of #17818 — same fix for bump_use() zero call sites, same target file (agent/skill_commands.py).

[alt-glitch]

Likely duplicate of #17818

[alt-glitch]

Likely duplicate of #17818

[alt-glitch]

Likely duplicate of #17818

[Giggitycountless]

Duplicate of #17818 — closing mine. Good to see this fix already in the pipeline! 👍