[dcadolph]

Two changes that were sitting uncommitted. Split into one commit each, both
building and passing on their own.

Name the files a commit touches in its subject

Subjects were built from the directory, so a commit adding a parser and its test
read Add api. That is a label, not a claim, and a reviewer cannot check it
against anything. They now come from the file list:

2. Add api/server.go and its test
     api/server.go
     api/server_test.go

3. Add store/db.go
     store/db.go

4. Update docs/guide.md
     docs/guide.md

Every subject is derived from the files and nothing else, which is what makes it
checkable against the commit it names.

Hold back generated output instead of committing it

A dirty tree carries build output and caches as often as it carries work, and
preen committed them because they were changes like any other. A __pycache__
directory landing in a commit is noise a reviewer reads past and a file the
project never wanted tracked.

Generated paths are now held back and named at the approval prompt, with the
pattern that caught each one:

Held back as generated output (1), left uncommitted:
     api/__pycache__/server.cpython-313.pyc  [__pycache__/: Python bytecode cache]
   Add them to .gitignore, or rerun with --allow-generated to commit them.

The plan says what it declined to commit rather than quietly staging it. A project
that keeps such a path deliberately says so in config under [generated], and
--allow-generated turns the check off for a single run.

Notes

go-cmp is a new test dependency, so go.mod and go.sum ride with the subject
commit, which is what needs them.

One imperfection worth naming: the README paragraph describing the new subjects
belongs with the subject commit and is in the generated-output commit instead. The
README hunks landed together when the two were separated, and splitting them
further was not worth rewriting the history for. Both land in the same merge.

Full suite passes, including the run package hook tests that were failing before
the test isolation fix on main.