chore: bump the pinned theme to v2.6.0

[mmcky]

Bumps the pinned lecture theme from v2.5.0 to v2.6.0, released 2026-09-10. One line in lectures/myst.yml; this site tracks every theme release, and no bot bumps a site.template URL.

What v2.6.0 brings

Change	Source
Code blocks highlighted in QuantEcon's own token palette (the Sphinx lectures' default), light and dark, mapped onto the highlight.js classes MyST emits	QuantEcon/quantecon-theme.mystmd#171
Emphasis, strong text and definition terms in the Sphinx builds' seoul256 colours	QuantEcon/quantecon-theme.mystmd#171
Language switcher, hreflang alternates, right-to-left support and a translator credit, all driven by new site options	QuantEcon/quantecon-theme.mystmd#174

This site sets none of the multilingual options, so the visible change here is the code and text colouring; the switcher and translator credit only render when languages / translators are configured. The same bump on lecture-python-programming#363 built green and the deployed stylesheet was checked for the new palette.

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[netlify]

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[Copilot]

🟢 Approval recommended

The change is a single, correctly-indented YAML URL update with no remaining references to the prior version.

Pull request overview

This PR updates the pinned QuantEcon MyST theme used by the lecture site by bumping the site.template download URL from v2.5.0 to v2.6.0, keeping the site aligned with the latest theme release.

Changes:

• Update lectures/myst.yml to pin the theme template URL to v2.6.0.
• Remove the previous v2.5.0 theme reference (no other references remain in the repo).

File summaries

File	Description
lectures/myst.yml	Bumps the pinned theme release URL from v2.5.0 to v2.6.0.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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[mmcky]

CI: build-and-deploy and data-url-guard pass on v2.6.0. The Netlify deploy-preview failure is pre-existing and unrelated to this bump: it has failed on every recent PR here, including the merged v2.5.0 (#72) and v2.3.0 (#67) bumps and the dataset cleanup (#68). Worth a separate look at the Netlify site config, but not a reason to hold this.