[mansortner]

What

• Adds an "Open in New Tab" entry to the Explorer context menu, offered on files, folders and empty space.
• Adds the t key as a shortcut.
• It opens a plain shell tab in the sidebar's workspace and focuses it. The tab's cwd is the selected folder, the parent of a selected file, or the root when nothing is selected, and the tab is labelled with the folder name.
• Implemented with tab.create over the IPC, then the existing viewer::focus_tab_for_client.
• Includes a unit test that the entry is offered for files, folders and empty space; the README key table has a new t row.

Why

You can already reveal a folder externally or preview a file, but there is no one-step way to start a terminal in the folder you are looking at.

t is free in Explorer; Source Control keeps t for its tree/list toggle, and the README row says so. Plain right-click is herdr's own pane menu, so the plugin menu is reached with m or Ctrl+right-click, and both reach the new entry.

Checklist

• cargo build --release succeeds (run from plugins/herdr-sidebar)
• cargo test passes (the flaky git::tests::ignored_scan_lock_is_single_flight also fails intermittently on untouched main, about 2 of 25 runs, so it is unrelated to this change)
• cargo clippy --release -- -D warnings is clean
• Exercised in a live herdr pane on herdr 0.8.2: t on a directory row opens a tab named after the folder, with the right cwd, focused; the menu entry works too
• README / AGENTS.md updated if keys, persistence, or the manifest changed (README key table)

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