docs(action.yml): top-level description says 'delete remote folder before' which is misleading

gh issue create --title "docs(action.yml): top-level description says 'delete remote folder before' which is misleading" --label "documentation,audit,priority:medium" --body "## Finding

action.yml's top-level description claims the action 'deletes the remote folder before' uploading. The actual behaviour, controlled by `delete: INPUT_DELETE`, only deletes when INPUT_DELETE is true. The description misleads users into thinking deletion is unconditional.

## Affected code

- action.yml — top-level `description:` field

## Reproduction

N/A — text review.

## Suggested fix

Rewrite the top-level description to:

'Deploys files to an FTP/FTPS server using lftp. Supports delete-before-sync (controlled by INPUT_DELETE), glob exclude patterns, server-side concurrency lock, and per-attempt retries with exponential backoff.'

## Source

F2 audit 2026-09-03 (action.yml audit). Reporter: subagent-pekodbj7.

## Related

- Part of EPIC #[epic-number]
- Closes: #NONE (no existing issue)"

Read the top-level description in action.yml alongside the delete: INPUT_DELETE setting. Update the description so deletion is clearly conditional and still mentions FTP/FTPS deployment, glob excludes, concurrency locking, and retries; done when the wording matches the documented behavior.

索引モデルが issue の本文から書いたものです。