feat: Make shell more robust

[veracioux]

I noticed a few things to improve in the current implementation of the shell
commands, so I went ahead and did it.

Changes

• Replaced scripts map with structured invocations map for clearer
  per-shell configuration
• Removed eval ${input.command} which is error-prone and not general
• Untangled bash and zsh rc file sourcing; previously it was sourcing both files
  for either shell, which could cause unintended side-effects

Verification

Ran the following:

SHELL=fish opencode
SHELL=sh opencode
SHELL=bash opencode
SHELL=zsh opencode
SHELL=nu opencode
SHELL= opencode (shows an error notification in the TUI, same as the current production version)

and within those TUI instances running the following command:

!echo "$SHELL" (!echo ($env.SHELL) for nu): prints the correct value

Tried the following edge cases in a random sample of shells:

! (empty shell command)
!cat
!bash
!| cat

None of those block, and have identical behavior as the production version.

[veracioux]

Rebased.

[rekram1-node]

thanks!