fix: preserve terminal and UTF-8 in sandbox plan editor

[Sayt-0]

Summary

• attach external editor processes to the real terminal streams so Vim no longer receives a pipe from Bubble Tea's image writer
• use the C.UTF-8 locale that the sandbox template actually provides, preserving German and other non-ASCII input
• cover both editor command construction and both sandbox CLI backends with regression tests

Issue expectations

Expectation	Implementation
Vim opens without the non-terminal warning while editing a plan in sandbox mode	external editor commands now use os.Stdin, os.Stdout, and os.Stderr directly
German and other non-ASCII input is preserved	sandbox execution and the published template use LANG=C.UTF-8
Both supported sandbox backends behave consistently	command construction is covered for docker sandbox and sbx

Root cause

The TUI configures Bubble Tea with an image-aware output writer. tea.ExecProcess populated an editor command's unset stdout with that writer, which os/exec exposed to Vim as a pipe rather than a terminal. Separately, the sandbox forced en_US.UTF-8, but the template only ships C, C.utf8, and POSIX; Vim consequently fell back to latin1.

Validation

• task test
• task --force lint
• task --force build
• real sbx microVM flow with the patched binary: /plans -> e -> Vim -> enter Viele Grüße äöüß -> :wq -> plan updated from v1 to v2 with exact UTF-8 content and no Output is not to a terminal warning

Closes #3873
Closes #3874