feat(v0.6): config file parser + resolver + source attribution

Phase 1 foundation. Adds:

- internal/config/configfile.go — strict-key YAML parser for the
  global config file. Unknown top-level keys invalidate the entire
  file (no partial apply); future schema versions are rejected with
  an upgrade message. Platform-appropriate path (XDG_CONFIG_HOME or
  ~/.config on Unix, %APPDATA% on native Windows).

- internal/config/resolver.go — Resolver / ResolvedSetting /
  SettingSource (Builtin, ConfigFileSrc, EnvVar, CLIFlag,
  PlatformOverride). Each setting carries provenance plus an
  Override chain so callers can render "env var (overrides config
  file: X)" lines. session_mode applies the native-Windows
  platform override at the resolver layer with the configured
  value chained as Override. Exports SetConfigPathForTest and
  SetIsNativeWindowsForTest so cross-package tests can isolate
  themselves from host config or simulate the override.

- internal/config/config.go — legacy ResolveRoot / ResolveAgent /
  ResolveSeedDir / ResolveProjectRoot / ResolveSessionMode now
  delegate to LoadResolver so config-file values take effect for
  entry commands. ResolveProjectRoot preserves the "" sentinel for
  built-in source so SearchRoots and doctor checkProjectRoot keep
  their existing fallback semantics. ResolveSessionMode preserves
  the v0.5.3 unknown-value stderr warning, distinguishing env-var
  and config-file sources in the message.

- Tests: 6 LoadConfigFile cases (missing, basic, partial, unknown
  key, future schema, malformed YAML), 3 ConfigFilePath cases
  (XDG / ~/.config / %APPDATA%), 11 resolver cases including
  layering, override chains, path expansion, platform override,
  and SettingSource string rendering.

- Test isolation: vulnerable tests in config_test.go and
  config_roots_test.go that asserted Builtin-source defaults now
  call SetConfigPathForTest to point at a nonexistent path,
  insulating them from developer-host config files. Three cmd
  tests that asserted persistent session_mode behavior now call
  SetIsNativeWindowsForTest to disable the platform override
  (Phase 1 introduces the override; the layering-only behavior
  these tests cover is tested separately in resolver_test.go).

No new dependencies (gopkg.in/yaml.v3 was already in go.mod).
No version bump (lands at the end of Phase 3 per the v0.6 spec).

internal/config/configfile.go

@@ -0,0 +1,119 @@
+package config
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"runtime"
+
+	"gopkg.in/yaml.v3"
+)
+
+// CurrentConfigSchemaVersion is the highest config-file schema version
+// this binary understands. A future schema version bump must add the
+// matching parser support alongside this constant.
+const CurrentConfigSchemaVersion = 1
+
+// ConfigFile is the strict YAML schema for the global ctask config file.
+// All fields are optional; missing fields fall back to built-in defaults
+// via the Resolver. Unknown keys invalidate the entire file (no partial
+// apply) per the v0.6 spec.
+type ConfigFile struct {
+	SchemaVersion   int    `yaml:"schema_version"`
+	CtaskRoot       string `yaml:"ctask_root"`
+	ProjectRoot     string `yaml:"project_root"`
+	SeedDir         string `yaml:"seed_dir"`
+	DefaultAgent    string `yaml:"default_agent"`
+	DefaultCategory string `yaml:"default_category"`
+	Editor          string `yaml:"editor"`
+	SessionMode     string `yaml:"session_mode"`
+}
+
+// configFileKnownKeys returns the set of YAML keys recognized by the
+// ConfigFile struct. Kept in lockstep with the yaml tags above.
+func configFileKnownKeys() map[string]struct{} {
+	return map[string]struct{}{
+		"schema_version":   {},
+		"ctask_root":       {},
+		"project_root":     {},
+		"seed_dir":         {},
+		"default_agent":    {},
+		"default_category": {},
+		"editor":           {},
+		"session_mode":     {},
+	}
+}
+
+// LoadConfigFile reads and parses the config file at path. Returns:
+//   - (nil, nil)  the file does not exist (caller falls back to defaults)
+//   - (nil, err)  any I/O, parse, unknown-key, or schema-version error
+//   - (cfg, nil)  on success
+//
+// Unknown top-level keys invalidate the entire file with a descriptive
+// error; ctask never partially applies a file with typos. Future schema
+// versions are rejected with an upgrade message.
+func LoadConfigFile(path string) (*ConfigFile, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, nil
+		}
+		return nil, err
+	}
+
+	// First pass: parse into a generic map of YAML nodes so we can
+	// inspect raw key names without trusting the typed-struct unmarshal
+	// (yaml.v3 silently drops unknown keys by default).
+	var raw map[string]yaml.Node
+	if err := yaml.Unmarshal(data, &raw); err != nil {
+		return nil, fmt.Errorf("parsing config file: %w", err)
+	}
+
+	known := configFileKnownKeys()
+	for key := range raw {
+		if _, ok := known[key]; !ok {
+			return nil, fmt.Errorf("unknown key: %q", key)
+		}
+	}
+
+	// Second pass: typed unmarshal.
+	var cfg ConfigFile
+	if err := yaml.Unmarshal(data, &cfg); err != nil {
+		return nil, fmt.Errorf("parsing config file: %w", err)
+	}
+
+	if cfg.SchemaVersion > CurrentConfigSchemaVersion {
+		return nil, fmt.Errorf(
+			"config file requires schema version %d, but this binary supports up to version %d. Please upgrade ctask.",
+			cfg.SchemaVersion, CurrentConfigSchemaVersion)
+	}
+
+	return &cfg, nil
+}
+
+// ConfigFilePath returns the platform-appropriate global config path.
+//
+// Linux / macOS / WSL:
+//   - $XDG_CONFIG_HOME/ctask/config.yaml when XDG_CONFIG_HOME is set
+//   - ~/.config/ctask/config.yaml otherwise
+//
+// Native Windows:
+//   - %APPDATA%\ctask\config.yaml (fallback: %USERPROFILE%\AppData\Roaming\ctask\config.yaml)
+//
+// The path is returned regardless of whether the file exists; callers
+// stat the path themselves.
+func ConfigFilePath() string {
+	if runtime.GOOS == "windows" {
+		appData := os.Getenv("APPDATA")
+		if appData == "" {
+			home, _ := os.UserHomeDir()
+			appData = filepath.Join(home, "AppData", "Roaming")
+		}
+		return filepath.Join(appData, "ctask", "config.yaml")
+	}
+	if xdg := os.Getenv("XDG_CONFIG_HOME"); xdg != "" {
+		return filepath.Join(xdg, "ctask", "config.yaml")
+	}
+	home, _ := os.UserHomeDir()
+	return filepath.Join(home, ".config", "ctask", "config.yaml")
+}