fix: treat missing slash commands as warning

[nettapper]

When parsing prompts, file paths should not be treated as slash commands and should not throw an error.

This PR downgrades missing slash commands to warnings, when linting missing slash commands are still treated as an error.

> go clean -testcache && go test ./...
ok  	github.com/kitproj/coding-context-cli	64.939s
ok  	github.com/kitproj/coding-context-cli/pkg/codingcontext	2.808s
ok  	github.com/kitproj/coding-context-cli/pkg/codingcontext/markdown	0.903s
ok  	github.com/kitproj/coding-context-cli/pkg/codingcontext/mcp	0.313s
ok  	github.com/kitproj/coding-context-cli/pkg/codingcontext/selectors	0.716s
ok  	github.com/kitproj/coding-context-cli/pkg/codingcontext/skills	0.588s
ok  	github.com/kitproj/coding-context-cli/pkg/codingcontext/taskparser	1.308s
ok  	github.com/kitproj/coding-context-cli/pkg/codingcontext/tokencount	0.470s