E2BIG Spawn Failure Diagnostics

When Claude Code cannot start a command because the OS argument/environment limit is exceeded (E2BIG), it now surfaces a detailed diagnostic instead of a generic spawn error:

Could not start bash: the command line plus environment exceed the OS exec argument
limit (E2BIG). At spawn: command line 12.3 KB across 47 args (largest single arg
4.1 KB); environment 38.9 KB across 312 vars (largest: MY_VAR at 2.1 KB).
The Bash sandbox profile adds 2847 filesystem deny paths to every command, 87 of
them for registered git worktrees, which grow this list without bound. From another
terminal, remove worktrees you no longer need (git worktree remove <path>; git
worktree prune for already-deleted checkouts), then restart Claude Code so the
profile is rebuilt without them — or relax the Bash sandbox for this session with
/sandbox.

The message reports command-line size, arg count, largest arg, environment size, var count, and largest env var. If the Bash sandbox deny list is a factor, it identifies how many paths come from accumulated git worktrees and gives remediation steps.