[Feature]: Collapse skill IPC results into native IpcApi errors

[kangfenmao]

Issue Checklist

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Platform

macOS

Version

main / PR #16060

Is your feature request related to an existing issue?

PR #16060 migrates skill commands from legacy preload IPC to IpcApi but intentionally preserves the existing SkillResult<T> success/failure envelope for compatibility during the migration.

That envelope now duplicates IpcApi's native error transport: handlers catch errors and return { success: false, error }, while callers either immediately rethrow through unwrapSkillResult() or branch on result.success where a normal try/catch would work.

Desired Solution

Follow up after PR #16060 by moving skill command failures to native IpcApi errors and removing the parallel SkillResult error channel where feasible.

Expected cleanup:

• Remove the skill-specific result wrapper schema if no longer needed.
• Remove or simplify unwrapSkillResult().
• Update install, uninstall, toggle, install_from_zip, install_from_directory, read_file, and list_files handlers/callers to use the standard IpcApi error model.
• Keep user-visible behavior stable.

Alternative Solutions

Keep SkillResult<T> permanently for these routes, but that would preserve a second error model on top of IpcApi and make future route migrations less consistent.

Additional Information

Tracked from PR review feedback on #16060.