refactor: update doc references and agent names to zensical…

[lisa-tarbo]

… from mkdocs

Resolves: dimagi/open-chat-studio#2544

[claude]

Nit: "Zensical" is treated as a proper noun everywhere else in this PR (README.md, write-docs.md's "Understanding the Zensical structure"), but it's lowercased here and again on lines 29–30 ("Follow zensical Best Practices", "zensical-specific extensions"). Worth capitalizing for consistency.

[claude]

Review summary

The rename is mechanically clean — no leftover mkdocs-technical-writer references, and the changelog template + command files are updated consistently. Two issues worth addressing before merge (left as inline comments):

1. README.md:36 — Note: This project uses the mkdocs.yml is a sentence fragment.
2. .claude/agents/zensical-technical-writer.md — "zensical" is lowercased in three spots (lines 8, 29, 30); other files in the same PR treat it as a proper noun.

Two things to verify manually:

• The new URL https://zensical.org/docs/authoring/markdown/ actually resolves and points to authoring guidance (couldn't fetch from this environment).
• .claude/commands/write-docs.md:18 still says "Updating mkdocs.yml navigation if needed" — accurate since the project does keep mkdocs.yml, but reads oddly next to the rebranding. Optional: clarify with "(Zensical uses the MkDocs config format)".

[Copilot]

Pull request overview

Updates this docs repo’s contributor guidance and Claude agent configuration to reference Zensical (the tool used to serve/build the site) rather than MkDocs-specific naming, aligning internal instructions with the current docs workflow.

Changes:

• Update README “Writing” link to the more specific Zensical authoring/Markdown docs and add a note explaining continued use of mkdocs.yml.
• Rename references from mkdocs-technical-writer to zensical-technical-writer in templates/commands.
• Update the Claude agent definition to use the new agent name and Zensical-oriented wording.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
README.md	Updates contributor-facing doc link and clarifies why mkdocs.yml is still used.
.github/templates/changelog-instructions.md	Switches documentation-agent instructions to zensical-technical-writer.
.claude/commands/write-docs.md	Updates the docs-writing command to launch the renamed agent and clarifies it targets user docs.
.claude/agents/zensical-technical-writer.md	Renames the agent and updates its description to reference Zensical instead of MkDocs.

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