Fix build command and errors.As usage per review feedback

- go build ./... compiles but does not produce a binary. Changed to
  go build -o gh-stack . which actually outputs the executable.
- errors.As(err, &ExitError{}) panics at runtime because the value
  type ExitError does not satisfy the error interface (only *ExitError
  does). Updated to the correct two-line pattern matching cmd/root.go.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: francisfuzz

.github/copilot-instructions.md

@@ -25,7 +25,7 @@ No Makefile, no code generation, no external linter config. Standard Go toolchai
 ## Coding conventions
 
 - Return typed `ExitError` sentinels (codes 1-10 in `cmd/utils.go`) from `RunE`. Never call `os.Exit()` directly.
-- Check errors with `errors.As(err, &ExitError{})`.
+- Check errors with `var exitErr *ExitError; errors.As(err, &exitErr)`.
 - Table-driven tests with `t.Run()` subtests.
 - Use `config.NewTestConfig()` for test configs with captured I/O.
 - Mock git: `restore := git.SetOps(&git.MockOps{...}); defer restore()`. Always defer restore.

AGENTS.md

@@ -89,7 +89,12 @@ Use typed exit codes defined in `cmd/utils.go`:
 | 9 | `ErrStacksUnavailable` | Stacked PRs not enabled for repository |
 | 10 | `ErrModifyRecovery` | Modify session interrupted |
 
-Return these from `RunE`. Never call `os.Exit()` directly from commands. Check with `errors.As(err, &ExitError{})`.
+Return these from `RunE`. Never call `os.Exit()` directly from commands. Check with:
+
+```go
+var exitErr *ExitError
+if errors.As(err, &exitErr) { ... }
+```
 
 ### Testing patterns