Insights and inspect commands for database diagnostics

Changelog

• d861cfcc862938059dfdfe7df1b764d4f204f6d8 Add insights and inspect commands for database diagnostics
• 19ee257842e6cdbe5fa4a58718edce84686b686f Fix inspect diagnostic accuracy
• 25cac7b41cee945e5e7cbeb9cd8c167d4ea69d61 Improve agent discoverability and support Vitess shard targeting
• 7d4f593cbee94e094a57db389c4dec54857e1ab2 Keep catalog next steps on checks that fail during inspect all
• 892881cc1d45d49e25b9ddc7a19939394615ad26 Only list tables with actual sequential scans in Postgres seq-scans
• d454630cd6ccc4c0864ac00d18ce436c57577d75 Print copy-pasteable next steps for skipped and matched checks
• b52b50900f54835c9220b6b8e1beb47fc64114a1 Sharpen PostgreSQL diagnostic queries from review feedback
• 9b09a05c32711e4655db92952cf6d43e59928f73 Use pg_table_size in table-sizes to avoid double-counting indexes