feat(terminal): resolve relative path links against the pane's current directory

A relative path printed in the terminal was resolved against the session's
workingDir, which is fixed at create time and never follows a cd. In a shell
that moved into a subdirectory or a sibling repo, the link opened the wrong
file or reported "File not found".

The pane-exit watcher's batched list-panes read already carries
#{pane_current_path}. derivePaneCurrentPaths() turns those rows into one
directory per single-pane session, TmuxManager.getPaneCurrentPath() serves it
from memory, and the server applies it on the existing paneExitsUpdated tick
as SessionState.cwd, broadcasting only on a change. No new tmux call, no new
SSE event. A failed read keeps the previous answers.

Session.setCwd() uses the paneExit scoping rule: remote and Docker sessions
never get a cwd (their local pane runs the ssh or docker exec client), nor do
direct-PTY or rediscovered sessions. The value is runtime-only.

In the browser, relativeResolutionBase() picks the live cwd when known and
workingDir otherwise. It only changes the base a relative path joins: the
result is routed as before, so a target outside the workspace still goes
through the attachment routes, and file-content/file-raw are unchanged.

src/mux-interface.ts

@@ -271,7 +271,7 @@ export interface PaneCaptureOptions {
  * - `sessionKilled` (data: { sessionId: string }) - Session terminated
  * - `sessionDied` (data: { sessionId: string }) - Session died unexpectedly
  * - `statsUpdated` (sessions: MuxSessionWithStats[]) - Stats refreshed
- * - `paneExitsUpdated` () - A pane read finished; ask `getPaneExit()` per session
+ * - `paneExitsUpdated` () - A pane read finished; ask `getPaneExit()` (and `getPaneCurrentPath()`) per session
  */
 export interface TerminalMultiplexer extends EventEmitter {
   /** Which backend this instance uses */
@@ -396,6 +396,16 @@ export interface TerminalMultiplexer extends EventEmitter {
    */
   getPaneExit?(muxName: string): PaneExit | undefined;
 
+  /**
+   * The directory the session's pane was in at the last pane read
+   * (`#{pane_current_path}`), or `undefined` when unknown. Read from the same
+   * batched watcher as {@link getPaneExit}, so it costs no tmux call. A relative
+   * path printed in the terminal resolves against it; it is never a
+   * confinement root. A multiplexer without it leaves relative paths resolving
+   * against `workingDir`.
+   */
+  getPaneCurrentPath?(muxName: string): string | undefined;
+
   /**
    * How many authoritative pane reads have agreed on the exit `getPaneExit()`
    * reports, or 0 when it reports none. The exited-agent sweep closes a session