file_sync: config-driven replacement for hardcoded auth sync

Replace the three hardcoded host→guest credential syncs (opencode,
claude, pi) with a generic `[[file_sync]]` config list. Default is
empty — users opt in to exactly what they want synced, with no
surprise about which tools banger "supports".

```toml
[[file_sync]]
host = "~/.local/share/opencode/auth.json"
guest = "~/.local/share/opencode/auth.json"

[[file_sync]]
host = "~/.aws"          # directories are copied recursively
guest = "~/.aws"

[[file_sync]]
host = "~/bin/my-script"
guest = "~/bin/my-script"
mode = "0755"            # optional; default 0600 for files
```

Semantics:
- Host `~/...` expands against the host user's $HOME. Absolute host
  paths are used as-is.
- Guest must live under `~/` or `/root/...` — banger's work disk is
  mounted at /root in the guest, so that's the syncable namespace.
  Anything outside is rejected at config load.
- Validation at config load: reject empty paths, relative paths,
  `..` traversal, `~user/...`, malformed mode strings. Errors name
  the offending entry index.
- Missing host paths are a soft skip with a warn log (existing
  behaviour). Other errors (read, mkdir, install) abort VM create.
- File entries: `install -o 0 -g 0 -m <mode>` (default 0600).
- Directory entries: walked in Go; each source file is installed
  with its own source permissions preserved. The entry's `mode` is
  ignored for directories.

Removed (all dead after this):
- `ensureOpencodeAuthOnWorkDisk`, `ensureClaudeAuthOnWorkDisk`,
  `ensurePiAuthOnWorkDisk`, the shared `ensureAuthFileOnWorkDisk`,
  their `warn*Skipped` helpers, `resolveHost{Opencode,Claude,Pi}AuthPath`,
  and the work-disk relative-path + default display-path constants.
- The capability hook registering the three syncs now calls the
  generic `runFileSync` once.

Seven tests exercising the old codepath deleted; six new tests cover
the new runFileSync (no-op on empty config, file copy, custom mode,
missing-host-skip, overwrite, recursive directory). Config-layer
test adds happy-path parsing and a case-per-shape table of invalid
entries (empty, relative host, guest outside /root, '..' traversal,
`~user`, bad mode).

README updated: replaces the "Credential sync" section with a
"File sync" section showing the new config shape.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/config/config_test.go

@@ -3,6 +3,7 @@ package config
 import (
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 
@@ -115,3 +116,92 @@ func TestLoadAppliesLogLevelEnvOverride(t *testing.T) {
 		t.Fatalf("LogLevel = %q, want warn", cfg.LogLevel)
 	}
 }
+
+func TestLoadAcceptsFileSyncEntries(t *testing.T) {
+	configDir := t.TempDir()
+	data := []byte(`
+[[file_sync]]
+host = "~/.aws"
+guest = "~/.aws"
+
+[[file_sync]]
+host = "/etc/resolv.conf"
+guest = "/root/.config/resolv.conf"
+mode = "0644"
+`)
+	if err := os.WriteFile(filepath.Join(configDir, "config.toml"), data, 0o644); err != nil {
+		t.Fatal(err)
+	}
+	cfg, err := Load(paths.Layout{ConfigDir: configDir})
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if len(cfg.FileSync) != 2 {
+		t.Fatalf("FileSync = %+v", cfg.FileSync)
+	}
+	if cfg.FileSync[0].Host != "~/.aws" || cfg.FileSync[0].Guest != "~/.aws" {
+		t.Fatalf("entry[0] = %+v", cfg.FileSync[0])
+	}
+	if cfg.FileSync[1].Mode != "0644" {
+		t.Fatalf("entry[1] mode = %q", cfg.FileSync[1].Mode)
+	}
+}
+
+func TestLoadRejectsInvalidFileSyncEntries(t *testing.T) {
+	cases := []struct {
+		name string
+		toml string
+		want string
+	}{
+		{
+			"empty host",
+			`[[file_sync]]` + "\n" + `host = ""` + "\n" + `guest = "~/foo"`,
+			"host path is required",
+		},
+		{
+			"empty guest",
+			`[[file_sync]]` + "\n" + `host = "~/foo"` + "\n" + `guest = ""`,
+			"guest path is required",
+		},
+		{
+			"relative host",
+			`[[file_sync]]` + "\n" + `host = "foo/bar"` + "\n" + `guest = "~/foo"`,
+			"must be absolute",
+		},
+		{
+			"guest outside /root",
+			`[[file_sync]]` + "\n" + `host = "~/x"` + "\n" + `guest = "/etc/resolv.conf"`,
+			"must be under /root or ~/",
+		},
+		{
+			"path traversal",
+			`[[file_sync]]` + "\n" + `host = "~/../secrets"` + "\n" + `guest = "~/secrets"`,
+			"'..' segments",
+		},
+		{
+			"tilde user",
+			`[[file_sync]]` + "\n" + `host = "~other/foo"` + "\n" + `guest = "~/foo"`,
+			"only '~/' is expanded",
+		},
+		{
+			"invalid mode",
+			`[[file_sync]]` + "\n" + `host = "~/x"` + "\n" + `guest = "~/x"` + "\n" + `mode = "rwx"`,
+			"must be octal",
+		},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			configDir := t.TempDir()
+			if err := os.WriteFile(filepath.Join(configDir, "config.toml"), []byte(tc.toml+"\n"), 0o644); err != nil {
+				t.Fatal(err)
+			}
+			_, err := Load(paths.Layout{ConfigDir: configDir})
+			if err == nil {
+				t.Fatalf("Load: want error containing %q", tc.want)
+			}
+			if !strings.Contains(err.Error(), tc.want) {
+				t.Fatalf("Load error = %v, want contains %q", err, tc.want)
+			}
+		})
+	}
+}