v0.0.18

feat: vocab learning mode (例文 + MP3 + 和訳/解説をクリップボードへ) (#4)

* feat: replace polly CLI with vocab learning mode

The CLI now takes an English word or phrase and:

1. Calls the GitHub Copilot SDK to generate an example sentence
   containing the term plus a Japanese translation and a short
   Japanese explanation, requested as a single JSON object.
2. Synthesizes an MP3 of the example sentence via AWS Polly.
   The filename is the example sentence with spaces, punctuation,
   and symbols replaced by underscores, runs of underscores
   collapsed, and a .mp3 extension appended.
3. Copies a formatted vocab card to the macOS clipboard via pbcopy.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* address review: tidy go.mod and hoist sanitize regex

- Run go mod tidy so copilot-sdk/go is a direct require.
- Hoist the underscore-collapsing regex to a package-level var
  for consistency with jsonObjectRe.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>