refactor!: make in2lambda a command group (convert)

[m-messer]

BREAKING CHANGE: file conversion moves from in2lambda <file> <filter> to in2lambda convert <file> <filter>.

cli is now a click.group so further subcommands (a wizard, etc.) can be added without overloading the top-level command. The former command body is unchanged, just relocated to convert; runner() is untouched.

• pyproject [tool.poetry.scripts] and command-line.rst still point at in2lambda.main:cli — sphinx-click renders the subcommand automatically.
• quickstart examples updated to in2lambda convert ... with a note.
• tests/test_cli.py added (CliRunner): help, output files, case-insensitive filter names, unknown-filter rejection.

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Stack: markdown-filter ← cli-group ← llm-client ← …
Base: markdown-filter — #23.

🤖 Generated with Claude Code

[peterbjohnson]

One thing to sort out before this merges: the old command form doesn't fail, it quietly does nothing.

Running against this branch with the locked click 8.1.7:

in2lambda convert <file> PartsSepSol     exit 0   (converts, as expected)
in2lambda /abs/path/example.tex PartsSepSol   exit 0   prints usage, converts nothing
in2lambda ./example.tex PartsSepSol           exit 0   prints usage, converts nothing
in2lambda example.tex PartsSepSol             exit 2   usage error

So anyone passing an absolute path, which includes every Docker invocation using /files/... and any script, gets a successful exit code, no output files and no error. A bare relative filename is the only form that fails properly.

Either forward the old form to convert with a deprecation warning, or make it exit non-zero with a message pointing at the new spelling. A test for the absolute-path case would be worth having, since that's the one that behaves worst.

Separately: this is a breaking change to a published CLI, but pyproject.toml still says version = "1.0.0" on every branch in the stack, and there's no changelog in the repo. Worth a major bump and a short migration note in the release, so anyone pinning the old form finds out why it stopped working.

[m-messer]

Added error message when command is not found. To my knowledge this isn't used elsewhere so no issues with rejecting the legacy command