check/diff: unclear error messages for multiple files, a directory, and empty inputs

[nao1215]

Three error-path UX issues found while writing atago E2E coverage. Each is a message-quality problem, not a crash; the exit codes are already correct.

1. check with two or more FILE arguments prints only usage, with no error line

omokage check one.md two.md   # stdout empty, exit 1, only the usage block

The zero-argument path says missing FILE; the too-many-arguments path (cmd/app.go default branch) calls flagSet.Usage() with no explanatory line, so the user cannot tell what was wrong. Expected: an explicit "check takes exactly one FILE" line, consistent with the rest of the tool.

2. check DIR leaks a raw Go error

omokage check ./corpus/   # "read ./corpus/: is a directory", exit 1

The train path has an IsDir branch; the check path passes the os.ReadFile error straight through. Expected: a formatted message like the URL/extension/missing-path messages (e.g. "check reads a single .md/.txt file; ./corpus/ is a directory — use train for directories").

3. diff of two empty files reports "Similarity: 100%" with a misleading feature message

: > e.md
omokage diff e.md e.md   # exit 0, Similarity 100%, diff line "no enabled features configured"

The real cause is that neither document has measurable text; the "no enabled features configured" fallback (internal/profile/profile.go) is wrong (features are enabled). Expected: a message that says the documents have no measurable content.