refactor(sdk): remove unreachable branches and dead helpers

[samzong]

Summary

Dead-code removal across all four SDK ports. No observable behavior change.

Each port carried a post-summary early return for zero installed/updated targets that no input could reach: the preceding guards already return when the plan is entirely empty and when any conflict or error is present, so installed + updated is necessarily positive at that point. Go carried an equally unreachable stdin branch, since the default reader is os.Stdin and the preceding *os.File type assertion already covers it.

The rest is dead weight:

• Go: bundleFile duplicated the exported SkillFile field for field; copySkillBundleDir had no production caller (its only use is inlined into the golden test).
• Rust: NormalizedSkillBundle.files was a copy of its BTreeMap values in the same order; normalize_bundle_path round-tripped a split/join that is the identity.
• TypeScript: unread label field, plus listSkillFiles and resolvePath with no remaining callers.
• Python: unread label field, plus dedupe, _has_install_writes, _coerce_output, a shadowing empty_install_report, and a _split_text helper equivalent to str.splitlines().

Bundle content hashes are unchanged. A base-versus-current probe over a five-file nested bundle inserted out of lexicographic order produces an identical digest in Rust, matching the untouched Python implementation, so the value persisted in .kitup.json is stable and cross-language parity holds.

Scope Check

• Stays within the v0.1 bundled-skill installer boundary.
• Does not add marketplace, registry, private remote install, script execution, GUI, MCP server, or agent runtime behavior.
• Updates golden cases for observable installer behavior changes. (No observable change; existing 89 cases pass unmodified across all four ports.)
• Regenerates host constants after spec/hosts.json changes. (spec/hosts.json untouched.)

Verification

• make check