feat(ai-ide, getting-started): guide the AI setup with a walkthrough

[ndoschek]

What it does

Moves the Theia AI onboarding out of the chat welcome message into a walkthrough on the welcome page, and opens the walkthrough mechanism to Theia extensions on the way.

@theia/getting-started: WalkthroughService built its walkthroughs exclusively from the contributes.walkthroughs section of deployed plugins, so a build-time Theia extension had no way to offer one, even though everything around it (cards, progress, completion events, walkthrough.open) is source agnostic.

• adds the WalkthroughProvider contribution point as a second, equal source
• both sources are collected before anything is pruned, so neither drops the other's walkthroughs
• Walkthrough.pluginId becomes optional, for walkthroughs that come from an extension rather than a plugin
• the welcome page moves its sections into a column of their own and lists up to five walkthroughs next to them, centred in a readable measure

@theia/ai-ide: the chat welcome message taught the AI features, gated the setup and picked a default agent all at once, while the welcome page carried a second, hard-coded AI banner in a package that knows nothing about AI. Guidance therefore only appeared once a user was already stuck in the chat, and nothing tracked what was already done.

• contributes "Get started with AI": AI support in the app, turn the features on, connect a language model, choose a default agent, ask a first question, tool confirmation and workspace trust, and a closing step on prompts, MCP, token usage, AI History and the experimental AI First layout
• steps complete on context keys describing what the user achieved (ai.hasReadyLanguageModel, ai.chat.hasRequest, ai.chat.hasDefaultAgent, ai.chat.toolConfirmationConfigured) rather than on settings changing, so a user who configured everything earlier finds them already ticked
• the opening step carries what the welcome page used to announce in its AI banner, including the opt-in and cost notice and the community links; the banner and news sections are removed
• the chat welcome keeps only the states it has to show in place (no model, AI disabled, workspace not trusted), each linking into the walkthrough
• adds ai.openGettingStartedWalkthrough ("Get Started with AI")

Contributes to #16730

How to test

With a fresh profile, or after running Help: Reset Walkthrough Progress:

1. The welcome page lists Get started with AI in the right column, 0 of 7. Open it.
2. Work through the steps. Each one ticks itself once you actually do it: enable the features, enter an API key, pick a default agent, send a chat message, change the tool confirmation default. The closing step is marked done by hand.
3. Progress survives a reload, and the card leaves the welcome page once every step is done. It stays reachable via More... and Help: Open Walkthrough.
4. Chat view: with no language model configured, and with the AI features disabled, the slim states appear and their links open the walkthrough.
5. Regression: a VS Code extension contributing contributes.walkthroughs still shows up, and installing it does not remove the AI walkthrough.

Follow-ups

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

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