fix(herdr): close Phase 4 item 2 — actually ask the agent for a handoff

Release already validated and uploaded a §6.1 handoff, but nothing ever
told the agent the .orchestra-handoff.json convention existed, so the
file it waited on never got written. rotate() now prompts the agent
once via a new optional herdr.HandoffRequester capability
(CLIAdapter.RequestHandoff) when the file is missing, and defers
Release until it appears, mirroring the .orchestra-report.md/B3 ask
pattern rather than inventing a handoff.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W1rkJ2hBMybnJctPbcy4tT

internal/herdr/herdr.go

@@ -153,6 +153,10 @@ type Session struct {
 	// session's lease was created — the immutable-spec hash continuity's
 	// pickup validation compares against on the next rotation (§6.2).
 	TaskFileSHA string `json:"task_file_sha,omitempty"`
+	// HandoffRequested is set once rotate() has prompted the agent to write
+	// its §6.1 handoff (HandoffFile) — avoids re-sending the same prompt
+	// every tick while Release keeps waiting for the file to appear.
+	HandoffRequested bool `json:"handoff_requested,omitempty"`
 }
 
 func (c *Client) Prompt(ctx context.Context, pane, text string, wait time.Duration) error {