feat: correctly include hosts from nested SSH Include

[ergoz]

Summary

This PR improves SSH host completion so entries from Include-d config files are reliably included in suggestions, not just hosts from the main config file.

What changed

• Updated zsh-ssh.zsh parser to handle Include directives more robustly:
  • Supports leading whitespace before Include
  • Uses case-insensitive matching for the directive
  • Keeps support for ~, relative paths, absolute paths, and glob expansion
• Added cycle protection for recursive include parsing:
  • Tracks already-visited config files (by normalized path)
  • Skips re-parsing files to prevent infinite recursion in cyclic include graphs
• Hardened multi-line config processing in completion pipeline:
  • Replaced unsafe echo $var usage with printf "%s\n" "$var" in key places
  • Preserves line breaks and block boundaries for AWK parsing
• Updated README.md with a concrete multi-file Include example (~/.ssh/config.d/*.ssh_config style)

Why this change

In some real-world setups, host definitions live in multiple files included from ~/.ssh/config. Completion was expected to include those hosts but could miss them in edge cases (or fail on recursive include loops). This PR aligns completion behavior with typical OpenSSH multi-file config layouts.

Validation

• Syntax check passed: zsh -n zsh-ssh.zsh
• Manual verification against a real user config with:
  • Multiple direct Include lines
  • Files under ~/.ssh/config.d/*.ssh_config
• Confirmed that sampled host aliases from each included file are present in _ssh_host_list output

Example include layout validated

Include ~/.ssh/config.d/work.ssh_config
Include ~/.ssh/config.d/home.ssh_config
Include ~/.ssh/config.d/company.ssh_config
Include ~/.ssh/config.d/amazon.ssh_config

# alsow works with

Include ~/.ssh/config.d/*.ssh_config

Notes

• No behavior changes were made to host extraction semantics beyond include handling robustness and safer line-preserving text flow.
• Existing markdown lint warnings in README.md are pre-existing and unrelated to this PR.

Also maybe fixes:

• Does Not Parse Includes When Using Symlink(s) #39
• Handle Include directive with absolute paths #29

[sunlei]

@ergoz Thank you.
However, there are some issues in it, and I will make adjustments after merging.