feat: define explore dashboards inline in metrics views via the explore: block

[nishantmonu51]

Makes the inline explore: block the primary way to get an explore dashboard for a version: 1 metrics view, and builds the editing experience around it:

• Parser: the inline explore: block now supports dimensions: and measures: field selectors (defaulting to all), and the emitted explore is flagged with defined_in_metrics_view.
• The metrics view workspace gets a three-way toggle (code / metrics editor / explore editor); the visual explore editor edits the explore: block of the same file next to a live dashboard preview.
• "Generate Explore Dashboard" now enables the inline explore instead of creating a separate explore file (create-and-preview-explore.ts is replaced by inline-explore.ts); generated metrics views (both AI and simple paths) include the explore: block out of the box.
• For metrics views with an inline explore, the header CTA shows Preview plus a "Go to dashboard" button that lists the metrics view's canvas dashboards and offers canvas generation.
• Updated AI agent instructions, JSON schema, docs, and e2e tests; added an e2e suite for inline explore editing and the new header CTA.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!