Add /dig steering skill

[stbenjam]

Adds user-invocable-only steering skills for when agent work is superficial or when its architecture needs a simple map.

Changes:

• Add /dig with the approved one-sentence skeptical investigation prompt.
• Add /map with the approved two-sentence implementation overview prompt.
• Preserve steering's non-implicit invocation policy and add matching metadata/icons.
• Link both skills from the steering README.
• Refresh generated docs and root skill links.
• Normalize the blank line after frontmatter in /dig.

Validation:

• make update
• make lint — 0 errors, 0 warnings; grade A
• git diff --check