[UX] `apc sync --dry-run` shows counts but not target file paths

[FZ2000]

Problem

Running apc sync --dry-run currently shows cache counts and then prints:

ℹ [dry-run] No files written.

This is not very useful. The whole point of --dry-run is to see exactly what would change: which files would be written or modified, and where.

A developer can't confidently run apc sync without knowing what it'll touch.

Suggested Fix

Enhance --dry-run to show a diff-like preview of what would be written:

$ apc sync --dry-run

  Sync Plan (dry-run — no files written)
  ───────────────────────────────────────
  Target: claude-code, cursor

  Skills (3)
    + write  ~/.claude/skills/typescript-expert/SKILL.md
    + write  ~/.cursor/rules/typescript-expert.mdc
    ~ update ~/.claude/skills/python-style/SKILL.md  (changed)

  MCP Servers (2)
    + write  ~/.claude/claude_desktop_config.json  (merge)
    + write  ~/.cursor/mcp.json  (merge)

  Memory (1)
    + write  ~/.claude/CLAUDE.md

  Run without --dry-run to apply.

This requires the appliers to expose a "plan" or "preview" mode, but would make --dry-run genuinely useful.