SKILL.md update in probe

[mathjams]

Tweaks the use-probe skill so it triggers on general code-location work, not only explicit "where is X" questions.

• description: reframed to "prefer probe for finding definitions, call sites, usages, config before a grep/find/glob sweep in an unfamiliar repo." The description is what the model reads to decide whether to invoke the skill, so this is the main change.
• body: "first pass when the user asks about project knowledge" → "default first step for code/doc discovery; prefer over grep/find/glob," and search "as the task evolves" rather than once.

Applied identically to both plugins/probe and plugins/probe-codex.