Reopening a file puts me back where I was in it #502

Context

Opening a file today always lands on line 1, whether it's a tab restored with its folder or a file closed and reopened minutes ago. This task makes TuiCode remember where you were in each file (cursor and scroll) and put you back there whenever a file opens without a place of its own to go to. Pitch: #217.

Prerequisite: #460 (second editor group) should merge first; it reworks the same open paths in Workbench.

Acceptance criteria

• Quit TuiCode (Ctrl+Q) with the cursor partway down a long file, then run tuicode on the same folder again: that tab opens with the cursor on the same line and column, scrolled the same way, and the status bar shows that Ln, Col.
• Every restored tab comes back at its own position, not just the active one: switching to another restored tab shows where you were in that one.
• Close a tab, then reopen the file from the Explorer or quick open: it opens where you left it.
• Switch folders with Open recent folder (or) and back: each folder's tabs come back at their positions.
• A file reached from a different folder (same path on disk) opens where you last were in it.
• Opening a file at a place still goes to that place, not the remembered one: a Find in files result, Go to definition, Usages, Go to line (gl), Enter in a diff, Back / Forward (gp / gn).
• If the file got shorter since (e.g. after a git checkout), the cursor lands on the nearest line that still exists, and nothing errors.
• A commit message or rebase todo that git hands TuiCode (anything inside .git) always opens at the top.
• Diff and Review tabs behave as they do today.

Tests

• The position store: save and load per file path, most-recently-used ordering, the cap on how many files it keeps (a few hundred), surviving a missing or corrupt file the way WorkspaceStateStore does, paths inside .git never recorded.
• The position is recorded on tab close, on folder change and on quit, and not on every keystroke.
• Opening without a target restores the position; opening with a target (each of the paths in the criteria) ignores it.
• Clamping when the remembered line or column is past the end of the file.
• Update the AGENTS.md note on workspace state that says cursor positions aren't persisted yet.

Out of scope

• Selections, multiple carets, word wrap or other per-tab view toggles: only the primary cursor and scroll.
• A Reopen closed tab command (I close a tab and want it straight back, but I have to find the file and my place in it again #343).
• Persisting Back / Forward history across restarts.
• Unsaved edits across a restart (hot exit).
• A setting to turn it off.
• Tracking where text moved when the file changed; clamping is enough.