AGT_SESSION_PWD from the scratch pane: report the scratch shell's own cwd

[parMaster]

A custom command fired from the scratch pane gets the main pane's directory in $AGT_SESSION_PWD, not the scratch shell's. The reference documents this ("the scratch terminal reports the main pane's, since it tracks no cwd of its own"), so it's not a bug, but it breaks the obvious use of the scratch pane.

Repro:

1. Session with an agent in the main pane, cwd ~/src/project

2. Open the scratch terminal, cd docs/plans, ls

3. Select a file name from the ls output and fire a custom command that resolves the selection against $AGT_SESSION_PWD, e.g.

   command "Open Selection" cmd+shift+w f=$AGT_SELECTION; cd "$AGT_SESSION_PWD" && [ -f "$f" ] && { ...open it...; exit; }; agtermctl session hud "no such file: $f" --hide-after 4 --target "$AGT_SESSION_ID" --socket "$AGT_SOCKET"
   

4. HUD says "no such file", because the command ran in ~/src/project, not ~/src/project/docs/plans

agtermctl tree --json shows the same thing: the scratch surface has a paneID but no cwd.

The scratch pane is where I browse files next to the agent, so selection-based commands are most useful from there. #482 fixed this for the split pane. Tracking OSC 7 for the scratch shell the same way would make $AGT_SESSION_PWD (and the cwd in the tree) correct when $AGT_PANE is scratch.

Workaround today is finding the shell by AGTERM_PANE_ID=$AGT_PANE_ID in ps -E and reading its cwd with lsof, which is too hacky for a one-line keymap command.

[umputun]

done in #981, will be in the next release.

• a custom command fired from the scratch gets the scratch shell's own directory in $AGT_SESSION_PWD, so your "Open Selection" example resolves against docs/plans
• agtermctl tree --json reports it as scratchCwd on the session node, while a scratch exists (shown or hidden)
• relative links clicked in the scratch resolve against the same directory
• on a session attached from another Mac the scratch is a local shell, so from there $AGT_SESSION_HOST is empty and the pair names that local shell

one thing stays as documented: running a command from the palette or with keymap run still uses the main or split pane, only a key chord pressed in the scratch picks up the scratch's directory.