dead-code: filter pub API surface by default (+ --include-pub flag)

[ajianaz]

What

cora dead-code counts public API of library crates as dead code.

Evidence

On codecoradev/uteke: 557 findings, majority are pub fn in uteke-core / uteke-mcp (library crates) whose purpose is to be consumed externally. Examples: chunk_markdown_embed_aware, consolidate, dream family.

Proposal

• Skip pub items by default (they are API surface, not dead)
• Add --include-pub to opt back in
• Alternatively: respect crate type ([lib] vs [[bin]]) from Cargo.toml

Impact

Combined with #519 this turns dead-code from 557 noisy findings into a small actionable set.