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/entry.go

@@ -63,6 +63,13 @@ func dispatchPersistent(hasTmuxSession bool, leaseState session.LeaseState) disp
 }
 
 func defaultRunWorkspaceEntry(opts WorkspaceEntryOptions) error {
+	// v0.6: if the resolver downgraded a configured persistent mode to
+	// direct via the native-Windows platform override, surface the fact
+	// to the user once before any launch work happens. attach
+	// (AlwaysPersistent=true) is excluded — it has no direct-mode
+	// fallback and refuses on native Windows via preflightPersistentEntry.
+	emitPlatformOverrideWarningIfNeeded(opts.AlwaysPersistent)
+
 	mode := config.ResolveSessionMode()
 	persistent := opts.AlwaysPersistent || (mode == "persistent" && !opts.Direct)