refactor: use named Permission field in Repository struct instead of anonymous embedding (#37441)

The `Repository` struct in `services/context/repo.go` embedded
`access_model.Permission` anonymously, causing all permission methods to
be promoted directly onto `Repository`. This made it unclear at call
sites whether a method belonged to `Repository` itself or to its
embedded `Permission`.

### Changes

- **`services/context/repo.go`**: Replace anonymous
`access_model.Permission` with named field `Permission
access_model.Permission`
- **49 files** updated to route permission method calls through the
named field:

```go
// Before
ctx.Repo.IsAdmin()
ctx.Repo.CanWrite(unit.TypeCode)
ctx.Repo.CanReadIssuesOrPulls(isPull)
slices.ContainsFunc(unitTypes, ctx.Repo.CanWrite)

// After
ctx.Repo.Permission.IsAdmin()
ctx.Repo.Permission.CanWrite(unit.TypeCode)
ctx.Repo.Permission.CanReadIssuesOrPulls(isPull)
slices.ContainsFunc(unitTypes, ctx.Repo.Permission.CanWrite)
```

Methods defined directly on `*Repository` (`CanWriteToBranch`,
`CanCreateBranch`, etc.) are unchanged.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: wxiaoguang <2114189+wxiaoguang@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Nicolas <bircni@icloud.com>

services/context/permission.go

@@ -15,7 +15,7 @@ import (
 // RequireRepoAdmin returns a middleware for requiring repository admin permission
 func RequireRepoAdmin() func(ctx *Context) {
 	return func(ctx *Context) {
-		if !ctx.IsSigned || !ctx.Repo.IsAdmin() {
+		if !ctx.IsSigned || !ctx.Repo.Permission.IsAdmin() {
 			ctx.NotFound(nil)
 			return
 		}
@@ -35,7 +35,7 @@ func CanWriteToBranch() func(ctx *Context) {
 // RequireUnitWriter returns a middleware for requiring repository write to one of the unit permission
 func RequireUnitWriter(unitTypes ...unit.Type) func(ctx *Context) {
 	return func(ctx *Context) {
-		if slices.ContainsFunc(unitTypes, ctx.Repo.CanWrite) {
+		if slices.ContainsFunc(unitTypes, ctx.Repo.Permission.CanWrite) {
 			return
 		}
 		ctx.NotFound(nil)
@@ -46,7 +46,7 @@ func RequireUnitWriter(unitTypes ...unit.Type) func(ctx *Context) {
 func RequireUnitReader(unitTypes ...unit.Type) func(ctx *Context) {
 	return func(ctx *Context) {
 		for _, unitType := range unitTypes {
-			if ctx.Repo.CanRead(unitType) {
+			if ctx.Repo.Permission.CanRead(unitType) {
 				return
 			}
 			if unitType == unit.TypeCode && canWriteAsMaintainer(ctx) {