Skip non-executable files, add .source suffix convention (#53)

[toumorokoshi]

Related: #53

Summary

Implements the design proposed in #53 (comment). Only files with the executable bit set are now recognized as tome commands, and a new .source suffix convention replaces the # SOURCE header for sourceable scripts.

This makes it possible to include READMEs, data files, and library scripts in a tome directory without them appearing in help output or tab completion.

Changes

• script.rs: is_tome_script now takes a &Path and checks: (1) no dot-prefix, (2) .source suffix files are always valid, (3) otherwise the file must have the executable bit. Added resolve_source_path for transparent .source file lookup. Removed # SOURCE header parsing — source detection is now purely filename-based.
• directory.rs: Passes full path to is_tome_script. Strips .source suffix from command names so foo.source appears as command foo.
• finder.rs: Uses resolve_source_path to find both foo and foo.source when resolving commands.
• complete.rs: Uses new is_tome_script(&path) for filtering, strips .source suffix from completion candidates, uses resolve_source_path for file lookup.
• Example files: Renamed source_example → source_example.source, source_example_fish → source_example_fish.source, use-arg → use-arg.source, navigate_directory → navigate_directory.source. Added non_executable_file test fixture. Removed # SOURCE headers from all files.
• Tests: Updated all tests for new paths and expectations. Replaced # SOURCE header tests with .source suffix tests.
• Docs: Updated authoring-scripts.md and directories.md to document the executable-bit requirement and .source suffix convention.

Breaking Changes

• Files without the executable bit are no longer recognized as commands (unless they have a .source suffix)
• The # SOURCE header comment is no longer recognized; use the .source file suffix instead