perf(sessioncmd): read every listed session's directory from one tmux call

[tfyl]

Why

A board with extensions installed spends most of a CPU core forking tmux. In a 60 s execsnoop of a live board with 149 sessions, the board process forked about 4,450 processes a minute. 2,073 of them were tmux list-panes -t gi_<id> -F #{pane_current_path}, and the board's tmux server sat at about 36% of a core answering them.

Summary

• An extension that lists the whole board on every poll pass (the 2 s poll_interval) goes through BoardList. For each running row, sessionInfo forked one list-panes to read the pane's directory. That came on top of the list-panes -a the listing had already run to see which rows are alive. With about 70 running sessions, that is 70 forks per pass, or 30 passes × 70 ≈ 2,100 forks a minute, which is the 2,073 measured.
• The liveness scan now reads #{pane_current_path} next to the pid (PaneScan.Paths), and the listing takes each running row's directory from it. Adopted panes, and a pane with no directory, still go through PaneCurrentPath as before.
• A directory name can contain a line break, and the scan would read the text after it as a pane line of its own. A pane's occupant could use that to forge liveness or a pid for any session. Each scan therefore starts every line with a random per-call mark. An unmarked line is treated as the rest of the directory above it, and that cut-short directory is dropped, so the old per-session read answers for it as before.

Numbers

TestBoardListReadsEveryDirectoryInOneTmuxCall counts the tmux processes one BoardList forks, through a logging shim, with 7 running sessions:

	tmux forks per listing
before	8 (1 + N)
after	1

On the board measured above, that removes the 2,073/min list-panes -t … pane_current_path line, about 47% of every fork the board made. Live execsnoop figures after deploy will be added in a comment.

Test plan

• TestBoardListReadsEveryDirectoryInOneTmuxCall: fails on main with 8 forks, passes here with 1; also checks that a pane which cd'd reports its new directory
• TestPaneScanIgnoresALineBreakInADirectory: a forged line after a line break neither adds a session nor overrides a pid, and a trailing space in a path is kept
• go test -count=1 ./internal/tmux/ ./internal/sessioncmd/ ./internal/extensionhost/ pass, run with env -u TMUX -u TMUX_PANE and a scratch TMUX_TMPDIR
• go test -short ./internal/ui/: passes except TestFocusedAdoptedPaneEchoesQuicklyWithNoClientOfItsOwn (257 ms against a 250 ms wall-clock budget, with two other suites running in parallel), which passes 3/3 run on its own
• Nothing the board draws changes, so this PR has no capture

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