Refactor: Split large files into one-file-per-declaration structure (#16)

[odrobnik]

Summary

Systematic refactoring to improve code navigation and maintainability by splitting large monolithic files into focused, single-responsibility files.

Changes

Phase 1: Models (commit 8e24a34)

• Split Models.swift into 11 individual files in Sources/PostServer/Models/
• One file per type: MessageHeader, MessageDetail, AttachmentInfo, etc.
• Zero logic changes

Phase 2: Commands (commit d926a7c)

• Extracted 22 command structs from PostCLI.swift into Sources/post/Commands/
• PostCLI.swift: 2,225 → 743 lines (-67% reduction!)
• Each command now has its own file

Phase 3: IDLE Extension (commit 2cc088d)

• Extracted ~820 lines of IDLE/Hooks logic into PostServer+IDLE.swift
• PostServer.swift: 2,126 → 1,326 lines (-38% reduction!)
• Changed private → internal for cross-file extension access

Helper Access (commit 73af797)

• Made helper methods internal for future extension use
• Preparation for additional extensions (Messages, Folders, Actions, etc.)

Impact

• ✅ 34 new files created (11 models + 22 commands + 1 extension)
• ✅ ~2,500 lines reorganized into focused files
• ✅ Fast navigation via CMD+O to any model, command, or feature
• ✅ Easier maintenance — smaller, focused files
• ✅ Better reviews — changes isolated to specific files
• ✅ Zero behavior changes — pure refactoring
• ✅ All tests pass (8/8)
• ✅ Zero new warnings

Before/After

PostCLI.swift:

• Before: 2,225 lines (all commands + shared utilities)
• After: 743 lines (main command + utilities only)

PostServer.swift:

• Before: 2,126 lines (all server logic)
• After: 1,326 lines (IDLE extracted, helpers exposed)

Testing

swift build -c release  # ✅ Success
swift test              # ✅ All 8 tests pass

Issue

Closes #16