implement `exec` as non-replacing command execution

[chaliy]

Problem

exec is intentionally excluded (TM-ESC-005) because it cannot replace the shell process in a sandbox. However, exec is commonly used in scripts to hand off to another command as the final action, and the current behavior (silent failure or error) breaks real-world scripts.

Impact

Scripts that use exec for plugin dispatch or command delegation fail entirely. Example from wedow/ticket (line 1351):

exec "$_plugin" "$@"

This is the plugin system's dispatch mechanism — without it, tk list, tk query, tk edit, and all plugin commands are broken.

Proposal

Implement exec as "run the command and exit with its exit code" — which is semantically equivalent in a sandboxed environment where there's no real process to replace:

exec some_command args...
# Equivalent to:
some_command args...
exit $?

This preserves the security invariant (no process replacement / sandbox escape) while supporting the common exec usage pattern. The exec with only redirections (exec 3>file) can remain unsupported or be a separate follow-up.

Security notes

• No actual process replacement — just run + exit
• FD-only exec (no command) can error explicitly
• Stays within sandbox containment (TM-ESC-005)
• Document the difference from real exec in specs

[chaliy]

Duplicate of #794 — both are already implemented. exec cmd args... runs the command and exits with its exit code via ControlFlow::Return.

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