feat: add uploadpack.allowFilter support for GRASP-01 compliance

Add mandatory uploadpack.allowFilter capability to support partial clones
and fetches as required by GRASP-01 specification. This enables efficient
git operations for bandwidth-constrained clients (e.g., browser-based git
clients like git-natural-api).

Changes:
- Add uploadpack.allowFilter=true to git subprocess configuration
- Update SmartGitServer test helper with filter support
- Add integration tests for filter capability advertisement and functionality
- Update documentation to reflect filter as required capability

Tests verify:
- Filter capability is advertised in info/refs
- Filtered clones with blob:none work correctly
- Filtered fetches with tree:0 work correctly

1 files changed, 5 insertions, 2 deletions

diff --git a/src/git/subprocess.rs b/src/git/subprocess.rs
index acee726..37fa382 100644
--- a/src/git/subprocess.rs
+++ b/src/git/subprocess.rs
@@ -33,13 +33,16 @@ impl GitSubprocess {
 
         let mut cmd = Command::new("git");
 
-        // GRASP-01 requirement: MUST include `allow-reachable-sha1-in-want` and
-        // `allow-tip-sha1-in-want` in advertisement and serve available oids.
+        // GRASP-01 requirement: MUST include `allow-reachable-sha1-in-want`,
+        // `allow-tip-sha1-in-want`, and `uploadpack.allowFilter` in advertisement
+        // and serve available oids and filtered requests.
         // These config options must be passed before the command name.
         cmd.arg("-c");
         cmd.arg("uploadpack.allowReachableSHA1InWant=true");
         cmd.arg("-c");
         cmd.arg("uploadpack.allowTipSHA1InWant=true");
+        cmd.arg("-c");
+        cmd.arg("uploadpack.allowFilter=true");
 
         cmd.arg(service.command_name());