feat(skill): tiny Filesmith skill that triggers on casual requests

[Maxaubert]

Closes #47.

• SKILL.md shrinks from 75 to about 30 lines: a table mapping what people say (compress this image, make this a webp, remove the background, merge these PDFs...) to the command, plus four rules (dry run for bulk, read the JSON, never run setup without asking, exit codes).
• The description lists everyday phrasings so agents pick it up without the word Filesmith.
• Event fields, the error-code table and the full safety rules moved to reference.md, which agents read only when needed. The drift guard test still checks reference.md names every command and flag.
• Every command in the cheat sheet was dry-run against the built CLI.
• Version 0.8.3 (merge after fix(release): the bundled-tool check can no longer hang #46, which is 0.8.2).

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