i am basically a singer and i don't know much about guitars but i'd like to develop into a songwriter. my question is how much guitar do i have to know if i want to write songs suitable for hard rock/metal? are learning chords and strumming enough? - i mean i can construct a vocal line over chords and ask the guitarist in my band to put riffs and solos under it. is this a good approach or it will limit the band's songwriting? or alternatively should i learn heavy metal rhythm playing?
Metal is generally more focused on riffs than on chords. If you change the riffs into chords, chances are you'll find yourself strumming Em half the time :-)

I think anyone with a musical skill thinking about picking up (another) instrument is making a right move. Your musicality will increase. I know I'm a better guitarist now that I have picked up bass.

But, to answer your question, it is possible to write metal and hardcore based on chords, though it is slightly unconventional as far as I know. It never hurts to try though - you might come up with some great stuff.

You might indeed also think about actually learning to play metal rhythm guitar yourself, if you have the time to practice and cash for gear. It's an incredible fun genre to play and you'd be able to add a lot more to your band's sound. I'm a bad singer for instance, but after lots of practice I can now sing backing vocals reasonably well and it really helps to make song parts stick out.
I make backing tracks for bass and guitar players. Check 'em out here.
try to transcribe your best metall/rock bands songs into playing chords. like try to play enter sandman with chords, not the riff and see what happens.
why not co-write with your guitarist? he can come up with a riff and you do the lyrics and vocal lines and so on...