perf(sessioncmd): answer a board read's liveness and directory in one tmux call

[tfyl]

Why

Board extensions read the sessions they watch on every poll pass, and each read forks tmux two or three times. In a 60 s execsnoop of a live board with 149 sessions, 1,345 forks a minute were tmux has-session -t gi_<id>, and every one of them was followed by a second fork for the same session.

Summary

• BoardGet called Exists (has-session) and then, for a live pane, PaneCurrentPath (list-panes -F #{pane_current_path}). BoardRead did both of those and also capture-pane. The has-session count matches that exactly: 828 Gets plus 517 Reads (the 517 capture-pane per minute in the same trace) is 1,345.
• The new Driver.LivePaneDir reads #{pane_id} #{pane_current_path} in one call. A pane id coming back counts as proof of life, the same test Exists already applies to an adopted pane, where display-message exits 0 with an empty line once the pane has closed. A live pane whose directory can't be read still counts as live and falls back to the row's directory, as before.
• runtime.currentInfo replaces every sessionInfo(x, driver.Exists(x.ID), …) in sessioncmd (get, mute, accounts, adopt, replace), and BoardRead uses it before capturing.
• This adds the same sessionInfoIn split as perf(sessioncmd): read every listed session's directory from one tmux call #301, hunk for hunk, so whichever of the two merges second merges cleanly.

Numbers

TestBoardGetAndReadForkOnceForLiveness counts the tmux processes each read forks, through a logging shim:

read	before	after
BoardGet, live session	2	1
BoardRead, live session	3	2
BoardGet, dead session	1	1

On the board measured above, that removes the 1,345/min has-session line, about 30% of every fork the board made. Live execsnoop figures after deploy will be added in a comment.

Test plan

• TestBoardGetAndReadForkOnceForLiveness: fails on main (2 and 3 forks), passes here; also checks that the reported directory follows a pane that cd'd, and that a dead row keeps its stored directory
• TestLivePaneDirAgreesWithExists: managed live, adopted live, adopted closed and never-created ids all give the same liveness as Exists
• go test -count=1 ./internal/tmux/ ./internal/sessioncmd/ ./internal/extensionhost/ and go test -short ./internal/ui/ pass, run with env -u TMUX -u TMUX_PANE and a scratch TMUX_TMPDIR
• Nothing the board draws changes, so this PR has no capture

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