[feature] Accept multiple --config paths and merge entries in the TUI

[JerryAgbesi]

What problem are you trying to solve?
Many users split their SSH config across several files — a personal ~/.ssh/config, a work config, per-project includes, generated files from other tooling, etc. Today skipper -c <path> takes exactly one file, so you can only browse one config source at a time. Switching context means re-running Skipper with a different -c, which defeats the purpose of having one fuzzy-searchable TUI over all your hosts.

Proposed solution
Allow -c, --config to be repeated (or accept a comma-separated list) and load all provided paths:

skipper -c ~/.ssh/config -c ~/work/.ssh/config -c ~/projects/foo/ssh_config

Behavior:

• Parse each path with the existing sshconfig.ParseHosts and concatenate the results in the order they were passed.
• Render the combined list in the TUI exactly as a single file is rendered today; fuzzy find spans the whole merged set.
• When no -c is passed, fall back to the current DefaultConfigPath() behavior (single file).
• Duplicate alias across files: surface both entries (do not silently drop), since they may differ in hostname/user/port. Consider showing the source file as a secondary label in the TUI to disambiguate — out of scope for MVP but worth noting.
• Errors: if any one file fails to parse, fail fast with an error that names the offending path (same ergonomics as today's single-file failure).

Alternatives considered

• Implement SSH's native Include directive support in the parser instead. That would solve part of this use case but relies on users adding Include lines to a primary config; Skipper should also work for users who want ad-hoc merging without editing their config. Not mutually exclusive — Include support could be a follow-up.
• A persistent "tracked configs" list managed by a skipper config add <path> command, as sketched in [Feature] Multiple config Files #6. That is a larger UX surface (stateful config about configs); this issue is the smaller, stateless version that just expands the -c flag.

Additional context

• Relates to [Feature] Multiple config Files #6 ("Multiple config Files"). [Feature] Multiple config Files #6 proposes tracking multiple config paths persistently and offering a skipper config view to list/switch them. This issue is the runtime-flag counterpart — no state, just "merge these files for this invocation." The two can coexist (persistent tracked list could become the default set of -c values), but they can ship independently. Maintainer should decide whether to close one as a subset of the other.
• Affected code: cmd/root.go (flag registration, resolveConfigPath, runRoot), internal/sshconfig/parser.go (no change needed — called once per path).
• Tests: extend parser_test.go with a merged-sources case, and add a cmd test for flag repetition.