chore: CONTRIBUTING guide, good-first-issues, and client.go decomposition

[jamiesun]

Two maintainability items surfaced by external review:

1. Bus factor = 1 (90 commits, single author). Add CONTRIBUTING.md (build/test workflow, sshx_e2e tag explanation, acceptance-matrix rule from docs/roadmap.md, PR expectations), label a handful of good first issues, and state a maintainer-succession intent in the README community section.
2. Large files drifting — internal/sshclient/client.go is 1113 lines mixing connection, auth, host-key, exec, and SFTP concerns; internal/app/config.go (812) and dryrun.go (792) are close behind. Split client.go along existing seams (connect/auth, hostkey, exec, sftp) with zero behavior change; existing tests must pass unchanged.

Acceptance

• CONTRIBUTING.md merged and linked from README
• client.go split into cohesive files, go test ./internal/sshclient green, coverage not reduced
• At least 3 issues labeled good-first-issue