feat: edit command

[rmarganti]

Adds a new edit subcommand and refactors the CLI to use proper subcommands.

Changes

edit subcommand

• Opens secrets from any source (URI or preset) in the user's $VISUAL/$EDITOR (falls back to vi)
• After editing, displays a git-style diff of changes and prompts for confirmation before writing back
• Supports editing a preset's from or to side via --target
• Handles parse errors gracefully by re-opening the editor
• Detects no-op edits and empty files, aborting early

Refactor: CLI structure

• init is now a proper subcommand instead of a magic --preset init value
• Extracted SyncArgs from Args to cleanly separate sync-specific flags (--from, --to, --preset, --diff) from global flags (--config) and subcommands
• Introduced Command enum (Init, Edit) for subcommand dispatch
• Renamed new_job → build_sync_job to clarify its purpose
• Simplified SyncArgs::validate by removing the special-case "init" check and unnecessary diff-mode branching

New dependencies

• tempfile for safe temporary file creation during editing