Feat/plain forge to installer

[kaja-s]

Summary

• The installer previously only set up the codeplain CLI, leaving plain-forge (spec-authoring skills) and the plyn editor extension as manual steps. Both are now offered inline during install.sh/install.ps1, with fallbacks when npx or a supported editor isn't found.
• Reworked color usage, spacing, and copy across install.sh, walkthrough.sh, and examples.sh (and their PowerShell equivalents) for a more consistent terminal experience.

New user flow

What a user now experiences running curl -fsSL https://codeplain.ai/install.sh | bash (or the PowerShell equivalent):

1. CLI install
   Installs uv if missing, then installs (or upgrades) the codeplain CLI via uv tool.

2. API key setup
   If CODEPLAIN_API_KEY is already set, asks whether to get a new one anyway. Otherwise prompts for sign-up (platform.codeplain.ai) and a key to paste in, then persists it to the shell rc file (or Windows user env vars).

3. Welcome screen
   Clears the screen, shows the ASCII art + "Sign in successful.", then asks: "? Would you like to get a quick intro to ***plain specification language?"

4. ***plain walkthrough (if accepted) — 5 screens, teaching order changed this session:

• (1/5) Overview of the four sections
• (2/5) Definitions
• (3/5) Implementation & Test Reqs
• (4/5) Functional Specs
• (5/5) Rendering code with codeplain

5. plain-forge step (new)
   Explains what plain-forge does, links the repo, asks "? Would you like to install it now?" — if yes and npx is found, runs npx plain-forge install inline (its own agent/scope picker takes over); if npx is missing, prints the manual command instead.

6. plyn editor extension step (new)
   Detects code/cursor on PATH. If found, asks to install the extension via --install-extension; if not found, just prints the marketplace link.

7. Example projects
   Same as before — offers to download plainlang-examples.

8. Final summary
   "You're all set!": a numbered list of whichever of the three primary next steps weren't already done (plain-forge install → plyn install → codeplain your-project.plain), then Discord + docs demoted to one muted line at the bottom, then "Happy development!"

[dusano]

Very nice improvement!