Support ?preset=<name> in workspace creation deeplink URLs #24029

Problem

The "Open in Coder" deeplink (/templates/<name>/workspace) supports pre-filling individual parameters via param.*, but there's no way to reference a preset by name.

This means sharing a deeplink for a preset requires manually encoding every parameter into the URL. These URLs are long, hard to maintain, and drift out of sync when the preset is updated in the template.

Proposal

https://coder.example.com/templates/my-template/workspace?preset=my-preset

The frontend already fetches presets for the creation page (CreateWorkspacePage.tsx#L80) — this would just look up the matching preset by name and apply its parameters as autofill values. The other URL query params are parsed nearby (L60-L63, L410-L434). Should compose with existing query params (mode=auto, param.* overrides, disable_params).

This discussion was drafted with assistance from Claude Code.