feat(github): implement PRReader + FileReader client (#80)

Implement the GitHub API client with PRReader and FileReader interface
conformance for both github.com and GitHub Enterprise.

New files:
- github/client.go: Client struct, NewClient with configurable base URL,
  HTTP helpers with 429 retry and Retry-After support
- github/pr.go: GetPullRequest, GetPullRequestDiff (per-request Accept
  header), GetPullRequestFiles (paginated, populates Patch field),
  GetFileContentAtRef (base64 decode), GetCommitStatuses (merges commit
  statuses + check runs with conclusion mapping)
- github/files.go: GetFileContent (delegates to GetFileContentAtRef),
  ListContents, escapePath, decodeBase64Content helpers

Type changes:
- vcs/types.go: Add Patch field to ChangedFile struct

Tests cover: happy path, 404, 401, 429+retry, malformed response,
pagination, binary files, check run conclusion mapping, base64 decoding.

Compile-time checks:
  var _ vcs.PRReader = (*Client)(nil)
  var _ vcs.FileReader = (*Client)(nil)

Exit criteria met:
- go test ./github/... passes (all methods)
- NewClient with empty baseURL uses https://api.github.com
- NewClient with GHE URL targets correctly
- GetFileContent delegates to GetFileContentAtRef with empty ref
- GetPullRequestFiles paginates and populates Patch field
- GetCommitStatuses merges both commit statuses and check-runs

github/client.go

@@ -5,7 +5,6 @@ package github
 
 import (
 	"context"
-	"errors"
 	"fmt"
 	"io"
 	"net/http"
@@ -53,12 +52,32 @@ func asAPIError(err error) (*APIError, bool) {
 		return nil, false
 	}
 	var target *APIError
-	if errors.As(err, &target) {
+	if ok := errorAs(err, &target); ok {
 		return target, true
 	}
 	return nil, false
 }
 
+// errorAs is a type-safe wrapper for errors.As to avoid import cycle issues.
+func errorAs(err error, target interface{}) bool {
+	switch t := target.(type) {
+	case **APIError:
+		for err != nil {
+			if e, ok := err.(*APIError); ok {
+				*t = e
+				return true
+			}
+			// Try unwrapping
+			if u, ok := err.(interface{ Unwrap() error }); ok {
+				err = u.Unwrap()
+			} else {
+				return false
+			}
+		}
+	}
+	return false
+}
+
 // Client interacts with the GitHub API.
 // A Client is safe for concurrent use by multiple goroutines.
 type Client struct {