feat: allow explanations in silent mode

[huyua9]

Problem

Silent mode skips the initial explanation, but it gives users no way to request an explanation for the generated script without revising it. The persisted SILENT_MODE=true setting also was not propagated to the revision flow.

What changed

• Add an Explanation action to the generated-script menu when silent mode is active.
• Reuse the explanation renderer for the initial, on-demand, and post-revision paths.
• Propagate the effective silent-mode setting from both -s and SILENT_MODE=true.
• Add executable regression checks for the menu condition and callback.

Closes #137

Validation

• ./node_modules/.bin/jiti tests/prompt-silent-mode.test.ts
• npm run typecheck
• npx prettier --check src/prompt.ts tests/prompt-silent-mode.test.ts
• npx eslint src/prompt.ts tests/prompt-silent-mode.test.ts
• git diff --check