Add a convert command to the command line

[peterbjohnson]

Every ticket writes its tests before its code, and its pull request reports a run over a target set or a corpus folder with the number of flagged fields. See docs/plan.md.

The command is in2lambda-agent convert DOCUMENT [--solutions FILE] [--filter FILE | --write-filter] [--out DIRECTORY]. It prints the report and writes the zip. The existing
run command keeps the spec route behind a flag, --route spec, and the README describes
convert as the way to convert a document. The ticket is done when the ME2 pair converts
from the command line with no flagged field.

---

Workbench ticket t38.

Live run of convert on the ME2 pair (manager, 2026-09-23 04:57)

solutions /Users/peterbjohnson/code/lambdafeedback/in2lambda-agent/ExampleContents/targets/ME2_Fluids_introduction/mech50010_fluid_mechanics_S1_20260921091254_s
flag      q2.p1.worked_solution: a stray minus sign inside or beside a display maths; Mathpix reads a separator line as one
flag      q3.p1.worked_solution: a stray minus sign inside or beside a display maths; Mathpix reads a separator line as one
fields    65 fields, route B did not run
build     /private/tmp/claude-501/-Users-peterbjohnson-code-lambdafeedback-in2lambda/94029b0b-e025-469e-ac13-85e39254e374/scratchpad/t38-out-b/mech50010_fluid_m