feat(charm): add lipgloss styling with Slack brand colors under charm experiment

[srtaalej]

Changelog

Adds lipgloss-based styling to all help output under the charm experiment flag, using a centralized Slack brand color palette shared across all CLI components, and migrates all direct Styler() usage to named style functions. 🦋

Summary

This PR introduces a centralized Slack brand color palette (colors.go) as the single source of truth for all styling across lipgloss, huh themes, and future bubbletea components

• Adds lipgloss-based styling to all CLI output functions (behind the charm experiment flag), alongside existing aurora styling
• Migrates all external Styler() call sites to use new style functions (Green(), Red(), Yellow(), Gray())
• Consolidates all legacy aurora code into one clearly marked block for easy removal when the charm experiment is permanently enabled

Test steps

make test testdir=internal/style  # all tests pass (including new TestStyleFlags and TestStyleFlags_CharmDisabled)
make test testdir=cmd/help  # all tests pass
make build 
./bin/slack --help -e charm  # flags show yellow names + gray descriptions, alias arrow uses ❱
./bin/slack project --help -e charm  # subcommand flags also styled (inherited help func)
./bin/slack run --help -e charm  # custom run help also styled (uses PrintHelpTemplate)
./bin/slack --help (no experiment)  # flags unchanged (plain text), arrow stays >
# Verify on both light and dark terminal backgrounds if possible

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 90.90909% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.50%. Comparing base (2ae86a3) to head (29b218e).

Files with missing lines	Patch %	Lines
internal/pkg/platform/activity.go	33.33%	4 Missing ⚠️
cmd/upgrade/upgrade.go	0.00%	2 Missing ⚠️
internal/pkg/apps/install.go	0.00%	2 Missing ⚠️
cmd/manifest/validate.go	50.00%	1 Missing ⚠️
internal/iostreams/printer.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #365      +/-   ##
==========================================
+ Coverage   65.27%   65.50%   +0.23%     
==========================================
  Files         216      216              
  Lines       17994    18065      +71     
==========================================
+ Hits        11745    11833      +88     
+ Misses       5160     5136      -24     
- Partials     1089     1096       +7     

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