fix: preserve directory scan boundaries

[soulteary]

Summary

• recognize valid SSH configuration files by parsing known OpenSSH directives instead of inspecting only six keywords in the first five lines
• ignore symlinks while walking configuration directories and support lines up to 1 MiB consistently
• insert a line boundary when concatenating files whose previous file lacks a final newline
• keep the over-limit scanner failure path covered

Verification

• go test ./...
• go test -race ./...
• go vet ./...
• git diff --check

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep Include-only files out of legacy directory conversion

When a scanned directory contains an Include-only top-level config—the exact case added by this change—the keyword lookup now adds that file to GetPathContent, which concatenates it with the discovered fragments. The normal conversion path then rejects the resulting Include directive in validateLegacySSHConfig with “Include cannot be represented”; lossless mode also rejects directory sources. Consequently, a common config containing Include config.d/* now makes the entire directory conversion fail, whereas it was previously skipped and the recursively discovered fragment files were converted.

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