fix(stats): clarify labels for code files and package breakdown

[flipvh]

cella stats used "source" for two things: the top line counted every scanned file as "source files", and the table then had a "source & config" category for just part of them.

• The top line is now code files and lists the extensions that count as code, taken from the same set the counter uses. The skipped count is no longer printed but stays in --json as skipped.
• The package list gets a dim source & config by package heading, since its counts and percentages cover that category only.
• The values on the first two lines now line up.

Before:

  source files  2699  (829 non-source skipped)
  lines of code 540.8k

After:

  code files     2699  (ts tsx js jsx mjs cjs json jsonc css html)
  lines of code  540.8k

    1979  73.3%  source & config      138.2k   25.6%
    ...
──────────────────────────────────────────────

  source & config by package
     914  46.2%  frontend              65.6k   47.5%

🤖 Generated with Claude Code