fix: support multiple -input flags in gen-docs

[chaptersix]

Summary

• Add stringSlice flag type to support multiple -input flags (standard flag package only keeps the last value)
• Pass inputs separately to ParseCommands which already supports variadic args
• Add test for multi-input generation
• Update CI to use make targets instead of raw commands

Test plan

• go test ./cmd/gen-docs/... passes
• make gen-docs generates docs correctly with both input files

Fixes the broken invocation where -input was specified twice but only the last file was used.

[stephanos]

(approved with understanding that make change is reverted)