Improve discoverability: SEO metadata, README, GitHub Pages

[s-johri]

Makes sshush easier to find via Google and GitHub search (the top sshush result today is an unrelated YAML config generator).

Already applied (repo settings, live now):

• Description rewritten with searchable keywords: Interactive terminal UI (TUI) for SSH key management — control ssh-agent, browse & edit ~/.ssh/config, connect to hosts, generate keys, and audit permissions. Written in Go.
• Topics: added ssh-agent, ssh-keys, ssh-client, golang, go, cli, terminal-ui, bubbletea, developer-tools, key-management; removed cmd, manager
• GitHub Pages enabled (main branch root → README renders as the project site); homepage URL set to https://s-johri.github.io/sshush/

In this PR:

• README: keyword-rich intro paragraph (SSH key manager / TUI / ssh-agent / config editor), release/CI/Go Report Card/license badges, and a disambiguation section pointing misdirected searchers to bencromwell/sshush
• _config.yml: gives the Pages site a proper <title>, meta description, Open Graph tags (jekyll-seo-tag), and sitemap.xml (jekyll-sitemap); excludes source dirs from the rendered site

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