Graphcore changes for natspec/autoprove

[jtoman]

1. Make the memory tool resilient to malformed paths, and not crash the whole pipeline
2. FS tool expansion
   1. Allows writing exclusion rules for FS/VFS tools using a lambda instead of a regex (what WAS I thinking?)
   2. Adds an "edit file" tool, modeled after the str_replace used by claude code, codex, etc. Much more token efficient for targeted edits
   3. Adds a notion of "global exclude", a predicate where the files are not just hidden from tools, but materialization. This is useful for when you don't want the agent to read/write the .git directory or copy its contents during materialization
   4. Adds a "layered" fs tools abstraction, effectively a union FS implement via FSBackends. The current fs_tools is simply an instantiation of this new layered abstraction with a single "DirBackend".