feat(v0.6): platform-override stderr warning on launch paths

Closes the half-feature surfaced during Phase 1 review: doctor now
shows that persistent session_mode is overridden to direct on native
Windows, but the launch-time entry paths were silently downgrading
without telling the user. This commit adds the one-line warning
specified by the user, gated to the four entry commands where the
downgrade actually has an effect.

cmd/persistent.go:

- New package-level const platformOverrideWarning carries the exact
  spec-mandated text so tests don't have to retype the copy.
- New helper emitPlatformOverrideWarningIfNeeded(alwaysPersistent).
  Loads the resolver, reads SessionMode(), and emits the warning to
  stderr when:
    1. the call site is NOT AlwaysPersistent (attach has no
       direct-mode fallback and continues to refuse on native
       Windows via preflightPersistentEntry), AND
    2. the resolver's value resolved through PlatformOverride.
  Doctor and info do not call this helper — they render source
  attribution themselves and an extra stderr line from a diagnostic
  command would be noise. "Once per invocation" is provided
  implicitly by the call site running at most once per ctask
  command; no warn-once subsystem.

cmd/entry.go:

- defaultRunWorkspaceEntry calls the helper at the top, before any
  launch work. This covers all four entry paths (new / resume / last
  / open) since they all funnel through runWorkspaceEntry. attach
  also routes here but sets AlwaysPersistent=true, so the helper
  short-circuits.

cmd/platform_warning_test.go (5 cases):

- TestPlatformOverrideWarningEmittedOnLaunch — simulated native
  Windows + config session_mode: persistent + AlwaysPersistent=false
  → warning text appears on stderr.
- TestPlatformOverrideWarningNotEmittedWhenDirect — config
  session_mode: direct → no warning even on simulated Windows.
- TestPlatformOverrideWarningNotEmittedWithoutConfig — builtin
  default (direct) → no warning.
- TestPlatformOverrideWarningNotEmittedOnNonWindows — persistent
  config but isNativeWindows() returns false → no warning.
- TestPlatformOverrideWarningSkippedForAlwaysPersistent — attach's
  AlwaysPersistent=true gate prevents emission. attach's actual
  refusal contract on native Windows is already covered by
  TestPreflightRefusesNativeWindows in persistent_test.go.

Validation: `go test ./... -count=1`, `go vet ./...`, `go build`,
and `just build-linux` all clean.

cmd/persistent.go

@@ -7,6 +7,7 @@ import (
 	"runtime"
 	"time"
 
+	"github.com/warrenronsiek/ctask/internal/config"
 	"github.com/warrenronsiek/ctask/internal/session"
 	"github.com/warrenronsiek/ctask/internal/shell"
 )
@@ -17,6 +18,38 @@ import (
 // defaultIsTTYCheck.
 var isTTYCheck = defaultIsTTYCheck
 
+// platformOverrideWarning is the exact stderr line emitted when the v0.6
+// session_mode platform override (persistent → direct on native Windows)
+// kicks in. Kept as a package-level string so tests can reference it
+// without re-typing the user-facing copy.
+const platformOverrideWarning = "[ctask] warning: persistent session mode is not supported on native Windows; using direct mode. Use WSL for persistent sessions."
+
+// emitPlatformOverrideWarningIfNeeded prints the platform-override
+// warning once per invocation when:
+//  1. the workspace entry is NOT marked AlwaysPersistent (attach has no
+//     direct-mode fallback and refuses via preflight rather than
+//     downgrading), AND
+//  2. the resolver's session_mode value resolved through the
+//     PlatformOverride source — i.e. the user asked for persistent mode
+//     via config or env but the host is native Windows.
+//
+// Doctor and info must NOT call this helper. They render source
+// attribution explicitly in their own output; emitting an additional
+// stderr line from a diagnostic command would be noise.
+//
+// "Once per invocation" is provided implicitly by the call site in
+// defaultRunWorkspaceEntry, which runs at most once per ctask command.
+// No warn-once subsystem is needed in v0.6.
+func emitPlatformOverrideWarningIfNeeded(alwaysPersistent bool) {
+	if alwaysPersistent {
+		return
+	}
+	s := config.LoadResolver().SessionMode()
+	if s.Source == config.PlatformOverride && s.Value == "direct" {
+		fmt.Fprintln(os.Stderr, platformOverrideWarning)
+	}
+}
+
 func defaultIsTTYCheck() bool {
 	return shell.IsTTY(os.Stdin) && shell.IsTTY(os.Stdout)
 }