feat: deny-by-default filesystem isolation

Flip the sandbox from allow-by-default reads (--ro-bind / /) to
deny-by-default (--tmpfs / with selective mounts). This makes the
sandbox safer by default — only system paths, CWD, and explicitly
allowed paths are accessible.

- Config: DefaultDenyRead is now *bool (nil = true, deny-by-default)
  with IsDefaultDenyRead() helper; opt out via "defaultDenyRead": false
- Linux: new buildDenyByDefaultMounts() using --tmpfs / + selective
  --ro-bind for system paths, --symlink for merged-usr distros (Arch),
  --bind for CWD, and --ro-bind for user tooling/shell configs/caches
- macOS: generateReadRules() adds CWD subpath, ancestor traversal,
  home shell configs/caches; generateWriteRules() auto-allows CWD
- Landlock: deny-by-default mode allows only specific user tooling
  paths instead of blanket home directory read access
- Sensitive .env files masked within CWD via empty-file overlay on
  Linux and deny rules on macOS
- Learning templates now include allowRead and .env deny patterns

internal/config/config.go

@@ -36,7 +36,7 @@ type NetworkConfig struct {
 
 // FilesystemConfig defines filesystem restrictions.
 type FilesystemConfig struct {
-	DefaultDenyRead bool     `json:"defaultDenyRead,omitempty"` // If true, deny reads by default except system paths and AllowRead
+	DefaultDenyRead *bool    `json:"defaultDenyRead,omitempty"` // If nil or true, deny reads by default except system paths, CWD, and AllowRead
 	AllowRead       []string `json:"allowRead"`                 // Paths to allow reading (used when DefaultDenyRead is true)
 	DenyRead        []string `json:"denyRead"`
 	AllowWrite      []string `json:"allowWrite"`
@@ -44,6 +44,12 @@ type FilesystemConfig struct {
 	AllowGitConfig  bool     `json:"allowGitConfig,omitempty"`
 }
 
+// IsDefaultDenyRead returns whether deny-by-default read mode is enabled.
+// Defaults to true when not explicitly set (nil).
+func (f *FilesystemConfig) IsDefaultDenyRead() bool {
+	return f.DefaultDenyRead == nil || *f.DefaultDenyRead
+}
+
 // CommandConfig defines command restrictions.
 type CommandConfig struct {
 	Deny        []string `json:"deny"`
@@ -417,8 +423,8 @@ func Merge(base, override *Config) *Config {
 		},
 
 		Filesystem: FilesystemConfig{
-			// Boolean fields: true if either enables it
-			DefaultDenyRead: base.Filesystem.DefaultDenyRead || override.Filesystem.DefaultDenyRead,
+			// Pointer field: override wins if set, otherwise base (nil = deny-by-default)
+			DefaultDenyRead: mergeOptionalBool(base.Filesystem.DefaultDenyRead, override.Filesystem.DefaultDenyRead),
 
 			// Append slices
 			AllowRead:  mergeStrings(base.Filesystem.AllowRead, override.Filesystem.AllowRead),