feat(stats): count what a branch changed by kind of file with --since

[flipvh]

cella stats counts the whole repository, so it cannot say where a pull request's lines went. This adds a branch
mode that can, as the tool behind a rule for cella's AGENTS.md: a PR that grows the codebase says which kind of
lines grew and why.

pnpm cella stats --since origin/main        # tables in the terminal, -V lists every file
pnpm cella stats --since origin/main --md   # a markdown table on stdout, for a PR description

Run on cella#1331, which shows as +722 −121 on GitHub:

  files    added  removed      net
      8     +438       -6     +432  tests
     14     +229     -109     +120  source
      1      +44        0      +44  docs
      2      +11       -6       +5  json
     25     +722     -121     +601  total

  source lines by content
            +152      -87      +65  code
             +49       -9      +40  comments
             +28      -13      +15  blank

  source lines by package
     11     +215      -95     +120  frontend
      3      +14      -14        0  backend

72% of that PR's growth is tests and 65 lines are new code, which the raw count hides.

How it counts

• Committed work only, compared with the merge base of HEAD and the ref: the same diff a pull request into that
  ref shows. Uncommitted files are reported as not counted.
• Every file in the diff counts. The snapshot only looks at code extensions; a diff also holds docs, workflows and
  lock files. So this adds docs (md, mdx) and other to the snapshot's categories and reads a lock file as
  generated. The snapshot's own classifier is reused unchanged for everything else.
• Source lines by content come from a zero-context patch. A diff shows changed lines only, so a comment is
  recognized by how its line starts (//, /*, *, {/*). The parser follows each hunk header's line counts, so a
  changed line that itself starts with +++ is not taken for a file header. The three parts add up to the numstat
  totals of the source files; a test checks that on a real repo.
• --md uses the existing machine-output mode: header, warnings and spinner go to stderr, the table to stdout.

Things to check

1. The flag is --since <ref>, not --diff. --diff <path> already prints one file's patch in analyze and
   contributions. The ref is required: a default would have to guess between main and origin/main, and a stale
   local main counts other people's commits as the branch's.
2. Overlap with refactor: code consistency pass against cella house style #41. That PR rewrites stats.ts, cli.ts, cella-cli.ts and types.ts. The mode lives in its
   own file, services/stats-since.ts, and touches those four by a few lines each (five export keywords, two option
   definitions, two config fields, one dispatch). Whichever merges second needs a small rebase.
3. It needs a cella config like every service, so it does not run inside this repo. The table below comes from
   calling collectBranchStats and formatMarkdown directly.

Verification

pnpm check clean. pnpm test: 282 passed, 27 files. New tests/stats-since.test.ts covers the classifiers, both
parsers on literal git output (binary files, quoted paths, a hunk line that looks like a header), and the collector on
a real repo: kinds, content split, package split, merge base after the ref moved on, uncommitted work left out. The
built binary was run against cella#1331's branch for the output above, plus the two error paths (--md without
--since, an unknown ref) and the unchanged snapshot mode.

Size

Kind	Files	Added	Removed	Net
Source	5	+396	-8	+388
Tests	2	+276	0	+276
Docs	1	+41	-2	+39
Total	8	+713	-10	+703

Source lines by content: code +281, comments +52, blank +55.

• Source is the mode itself, in one new 366-line file: two parsers (numstat, patch), the collector, and two
  formatters (terminal tables, markdown). The shared files grow by 30 lines of wiring.
• Tests are one new file for the mode and one parse case in cli.test.ts.
• Docs is the README section with the kinds table and the comment heuristic's limit.

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