Every New Yorker has a favorite decompression chamber: The Conservatory Garden (Fifth Avenue and 103rd Street), with its lush, undulating hedges and wisteria pergola; the tiny but charming Sutton Place Parks, at the eastern end of 55th and 57th Streets; the relatively undiscovered courtyard at the General Theological Seminary, on Ninth Avenue at 20th Street. But you should know about an obscure slice of nirvana called The Garden At Saint Luke's (the church at 487 Hudson Street, at Grove Street; 924-0562). Even on a bad day, this serene patch of greenery could pass for a Merchant-Ivory location. Thanks to vine-covered, heat-retaining brick walls, the garden enjoys an unusually temperate microclimate: Several species of pomegranate, rosemary, and fig thrive here, things rarely seen north of the Carolinas. And the garden is a popular rest stop for migrating species. Visitors scribble notable sightings in a diary kept in a Ziploc bag. But even in an Eden such as this, there is profound pathos. Diary entry, 4/3/98: Discovered dead Y. B. sapsucker (male) today -- R.I.P.