pi: the session name in herdr's Agent sidebar

herdr labels every pi pane "pi", so with several open the rows cannot be told
apart. herdr 0.9.3's socket API has pane.report_metadata, whose display_agent
replaces that label without touching lifecycle state; this reports the session
name there on start and on every rename, auto-named titles included.

Interactive sessions only: a pi -p or subagent run from inside a pane inherits
its HERDR_* variables and would otherwise rename the pane after itself. Checked
against a fake herdr socket: a print-mode run sends nothing, while the socket
accepted a control message. Outside herdr it returns at once. The pi APIs it
uses -- ctx.mode "tui", getSessionName, session_info_changed's name -- are as
in pi 1.0.0, and link/pi.sh links it in like any tracked extension.

From change request e053eb.

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