Fix Television channel parse errors when the user's shell is nushell

[nuTTeLLo]

Problem

Termscope's Television cable channels use POSIX shell syntax:

[source]
command = [
  { name = "Appearance", run = "cat \"$TERMSCOPE_APPEARANCE_CANDIDATES\"" },
  ...
]

[preview]
command = "\"$TERMSCOPE_PYTHON\" \"$TERMSCOPE_SCRIPT\" preview --pane-path \"$TERMSCOPE_PANE_PATH\" {split:\t:0}"

Television runs channel source/preview/action commands through the shell detected from $SHELL on Unix (unless a channel overrides it). For users whose login shell is nushell, the picker's source and preview both break with:

Error: nu::parser::parse_mismatch
x Parse mismatch during operation.
  -[source: 1:21]
1 | "$TERMSCOPE_PYTHON" "$TERMSCOPE_SCRIPT" preview --pane-path "$TERMSCOPE_PANE_PATH" ...
  AAAA
  - expected operator

nushell doesn't parse the POSIX-style $VAR expansion/quoting the way a POSIX shell does.

Fix

Declare shell = "bash" on the [source] and [preview] sections of both built-in cable channels (termscope-appearance.toml and termscope-alpha.toml). These channels always emit POSIX syntax (they rely on $TERMSCOPE_* env expansion), so they should explicitly request a POSIX shell for the commands they run, matching how npm.nix-style channels already override the channel shell.

This is forward-compatible: bash exists on macOS and Linux (termscope's supported platforms), and the channels already depend on bash-isms in the form of $VAR env expansion.

Testing

Reproduced the nu::parser::parse_mismatch error under nushell, then verified that after adding shell = "bash":

• the channel TOML still parses and Television still accepts the channel;
• running the source/preview commands works under bash with the $TERMSCOPE_* env vars set;
• launching the picker on the cable directory no longer produces a nushell parse error.

Fixes termscoper failing for nushell users.