Add inline S3 completions for aws s3 command

Replace the non-functional loadSpec reference for aws s3 with inline
subcommand definitions for all 9 S3 subcommands (cp, ls, mb, mv,
presign, rb, rm, sync, website) with their full options and argument
suggestions.

Add s3_buckets generator that parses 'aws s3 ls' output to provide
dynamic bucket name completions for ls, rb, and website subcommands.

Add unit tests for parse_s3_buckets covering typical output, empty
output, dotted bucket names, and malformed lines.

Co-Authored-By: Oz <oz-agent@warp.dev>

Author: oz-agent