fix(apply): propagate read errors, auto-open repo, dedupe adapter base (#6)

Three robustness fixes for the apply pipeline:

1. Silent read errors on state/target files treated 'file absent' as
   'file empty' and fast-forward-overwrote user edits. Add
   common.ReadExistingFile that returns (nil,nil) only on
   os.IsNotExist; other I/O errors propagate. Use it from apply.Run
   and from every adapter's Render (claudecode, crush, kilo,
   opencode, goose, zed).

2. apply.Run required callers to pre-construct *gitx.Repo; passing
   nil silently skipped the commit. Run now auto-opens a repo from
   Bundle.Root, gracefully no-ops when the root is not a git repo,
   and uses a new gitx.HasStagedChanges() check to skip the no-op
   'nothing to commit' that triggered the original silent-skip.

3. Each of 7 adapter packages re-implemented the same 25-line
   Option/WithHome/New constructor. Hoist into adapter.Base +
   adapter.BaseOption + NewBase. Each adapter embeds *Base and
   reduces to a 9-line constructor. Public New(WithHome(h)) API
   preserved; a.home becomes a.Home.

Tests:
  - apply: 5 new (propagates-read-error, auto-opens-repo,
    no-repo-skips-commit, skips-no-op-commit, commit-message-counts)
  - gitx: 2 new (HasStagedChanges empty/after-add)
  - adapter/common: 3 new (ReadExistingFile missing/content/perm)
  - adapter: 4 new (NewBase default/WithHome/order/typed)
  - existing adapter tests unchanged — verify a.Home still honoured.

internal/adapters/cagent/cagent.go

@@ -19,21 +19,16 @@ import (
 const name = "cagent"
 
 // Adapter implements adapter.Adapter for the cagent harness.
-type Adapter struct{ home string }
+type Adapter struct {
+	*adapter.Base
+}
 
-// Option configures an Adapter.
-type Option func(*Adapter)
-
-// WithHome overrides the home directory (defaults to os.UserHomeDir).
-func WithHome(h string) Option { return func(a *Adapter) { a.home = h } }
+// WithHome overrides the home directory used to resolve target paths.
+func WithHome(h string) adapter.BaseOption { return func(b *adapter.Base) { b.Home = h } }
 
 // New returns a new Adapter with the given options applied.
-func New(opts ...Option) *Adapter {
-	a := &Adapter{home: common.DefaultHome()}
-	for _, o := range opts {
-		o(a)
-	}
-	return a
+func New(opts ...adapter.BaseOption) *Adapter {
+	return &Adapter{Base: adapter.NewBase(opts...)}
 }
 
 // Name returns the harness identifier.
@@ -53,14 +48,14 @@ func (a *Adapter) Capabilities() adapter.HarnessCapabilities {
 
 // Detect returns true when ~/.config/cagent/ exists.
 func (a *Adapter) Detect() bool {
-	_, err := os.Stat(filepath.Join(a.home, ".config", "cagent"))
+	_, err := os.Stat(filepath.Join(a.Home, ".config", "cagent"))
 	return err == nil
 }
 
 // Render produces a starter default.yaml for cagent.
 func (a *Adapter) Render(b *canonical.Bundle) (*adapter.FileSet, error) {
 	fs := adapter.NewFileSet()
-	cfgPath := filepath.Join(a.home, ".config", "cagent", "default.yaml")
+	cfgPath := filepath.Join(a.Home, ".config", "cagent", "default.yaml")
 
 	instructions := b.Instructions.Global
 	if override, ok := b.Instructions.PerHarness[name]; ok && override != "" {