refactor: remove survey package and permanently enable huh prompts

[srtaalej]

Changelog

The internal/iostreams/survey.go file has been deleted, removing all survey-based prompt implementations, custom templates, color helpers, and the SurveyOptions configuration function. The five prompt methods in prompts.go (ConfirmPrompt, InputPrompt, MultiSelectPrompt, PasswordPrompt, SelectPrompt) no longer check experiment.Huh and instead call their huh form counterparts directly.

Summary

This PR completes the migration from the survey library to Charm's huh package for interactive prompts.
The experiment.Huh flag and all survey-based prompt code have been removed, making huh the permanent and only prompt implementation.
The ThemeSurvey() theme (introduced in the parent PR) ensures visual continuity by matching the legacy survey styling when the Lipgloss experiment is off.

Test Steps

make test testdir=internal/iostreams   # and verify all prompt form tests pass including the ThemeSurvey and ThemeSlack theme assertions
make test testdir=internal/style   # and confirm TestThemeSlack, TestThemeSurvey, and TestChevron pass without the removed TestSurveyIcons
make test testdir=internal/experiment    # and confirm the Test_Includes assertion no longer expects "huh" to be a registered experiment
make test testdir=cmd/project    # to verify the sample selection flow works without experiment gating
make lint     # to confirm zero issues
slack create    # interactively and verify prompts render with the expected cursor, selection indicators, and color scheme
slack env add MY_VAR    # without a --value flag and verify the mimicked input line still appears before the password prompt

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

❌ Patch coverage is 74.50980% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.31%. Comparing base (1065713) to head (d1e7e59).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/iostreams/prompts.go	0.00%	12 Missing ⚠️
cmd/project/create_samples.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #425      +/-   ##
==========================================
+ Coverage   69.72%   70.31%   +0.58%     
==========================================
  Files         220      219       -1     
  Lines       18446    18335     -111     
==========================================
+ Hits        12862    12892      +30     
+ Misses       4410     4275     -135     
+ Partials     1174     1168       -6     

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