reland: portable startup-shell dialect, with the two revert causes fixed

Relands #14863 (reverted by #14975) with fixes for both regressions the
revert cited.

1. History GC deleted folder-workspace shell history. The live set was built
   from `getAllWorktreeMeta()` alone, but a folder workspace's PTY carries
   `folder:<id>` as its worktree id, so every live folder workspace looked
   orphaned. `getKnownWorktreeIdsForHistoryGc` now unions in
   `getFolderWorkspaces()`. Both consumers — the history-directory prune and
   the fish-history sweep — read that one set, so the fix covers bash, zsh and
   fish history alike. The directory prune had this gap since #1524; #14863
   only widened its blast radius to fish files.

2. A copied Codex resume command aborted under `set -u`. Its leading clear
   statement has to test `$fish_pid`, and that unbound expansion takes the
   whole line — including the agent launch — down with it. Copied text runs in
   a shell Orca never spawned, so nothing can seed that variable first. The
   removal now rides on the agent itself as `env -u`, which needs no shell
   syntax and no expansion. Verified byte-identical under `set -u` in sh,
   bash, zsh, dash, ksh and fish.

   `env` cannot run the `cd` builtin, and a child `cd` would not move the
   agent, so the prefix is placed on the agent rather than on the whole
   `cd … && agent` chain. cmd and PowerShell have no nounset hazard and keep
   their clear ahead of the `cd`, which preserves `cd … && agent` — a failed
   `cd` still cannot launch the agent in the wrong directory.

.github/workflows/pr.yml

@@ -270,7 +270,10 @@ jobs:
             src/main/providers/local-pty-shell-ready-zsh-zdotdir-normalization.test.ts \
             src/main/providers/__tests__/shell-ready-framework-example.test.ts \
             src/main/pty/omp-shell-wrapper.node-pty.test.ts \
+            src/main/terminal-history-fish-session.node-pty.test.ts \
+            src/main/zsh-scoped-histfile.live-shell.test.ts \
             src/renderer/src/components/terminal-pane/fish-color-scheme-child-stdin.node-pty.test.ts \
+            src/shared/startup-shell-portability.live-shell.test.ts \
             src/shared/posix-command-path-lookup.test.ts
 
   test:
@@ -309,7 +312,10 @@ jobs:
             --exclude=src/main/providers/local-pty-shell-ready-zsh-zdotdir-normalization.test.ts \
             --exclude=src/main/providers/__tests__/shell-ready-framework-example.test.ts \
             --exclude=src/main/pty/omp-shell-wrapper.node-pty.test.ts \
+            --exclude=src/main/terminal-history-fish-session.node-pty.test.ts \
+            --exclude=src/main/zsh-scoped-histfile.live-shell.test.ts \
             --exclude=src/renderer/src/components/terminal-pane/fish-color-scheme-child-stdin.node-pty.test.ts \
+            --exclude=src/shared/startup-shell-portability.live-shell.test.ts \
             --exclude=src/shared/posix-command-path-lookup.test.ts \
             --exclude=tests/e2e/cross-version-wire/** \
             --shard=${{ matrix.shard }}/${{ matrix.shard_total }}