feat(init): support sanity-template.json with postInitMessage

[hermanwikner]

Description

This pull request lets template authors include an optional sanity-template.json as part of the template, with a postInitMessage that the CLI shows after the template has been successfully set up. This allows template authors to highlight next steps, links, commands or anything else they want people to see at the end of init – maintained in the template itself, so copy tweaks don't require a new CLI release.

Example flow

1. Given a template that contains sanity-template.json at the project root, e.g.:

   {
     "postInitMessage": "POST MESSAGE"
   }

2. When the user bootstraps that template, e.g:

   sanity init --template <your-template-ref> --output-path /path/to/project

3. Then they see the usual success output from init and after that the postInitMessage appears, e.g:

   Success! Your Studio has been created.
   
   (cd /path/to/project to navigate to your new project directory)
   
   Get started by running pnpm dev to launch your Studio's development server
   
   Other helpful commands:
   npx sanity docs browse     to open the documentation in a browser
   npx sanity manage          to open the project settings in a browser
   npx sanity help            to explore the CLI manual
   
   Message from the template author:
   
   POST MESSAGE
   

What to review

• From the repo root, after a local build, run sanity init against a template branch that includes sanity-template.json with postInitMessage – e.g. this branch of the Next.js clean starter where that file was added:

  node packages/@sanity/cli/bin/run.js init \
    --template https://github.com/sanity-io/sanity-template-nextjs-clean/tree/herman-test \
    --output-path /tmp/test-template

• Confirm the postInitMessage appears after the usual success output

[claude]

Claude finished @hermanwikner's task in 1m 14s —— View job

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PR Review

• Gather context
• Read changed files
• Provide review feedback

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Issues

1. sanity-template.json not removed when manifest exists but fails schema validation

removeTemplateManifestFromOutput is only called when templateManifest is truthy (init.ts:737). If the file exists but fails schema validation (e.g. postInitMessage: 42), readTemplateManifest returns null, and the file is left in the user's project directory. The cleanup should be unconditional — removeTemplateManifestFromOutput already swallows errors if the file doesn't exist.

// current
if (templateManifest) {
  await removeTemplateManifestFromOutput(outputPath)
}

// fix: always attempt cleanup, regardless of parse result
await removeTemplateManifestFromOutput(outputPath)

To support this, readTemplateManifest would need to indicate "file existed but was invalid" vs "file didn't exist" — or simply always call removeTemplateManifestFromOutput unconditionally (it's already safe to do so). Fix this →

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2. Inconsistent size limits in the schema

In types.ts:23:

z.union([z.string().max(2000), z.array(z.string().max(500)).max(50)])

String form caps at 2,000 chars, but array form allows up to 50 × 500 = 25,000 chars. A template author using the array form can display 12× more text, bypassing the intended limit. Consider either making the total-character limit consistent across both forms, or adding an overall character budget to the array branch. Fix this →

[github-actions]

Coverage Delta

File	Statements
packages/@sanity/cli/src/actions/init/constants.ts	100.0% (±0%)
packages/@sanity/cli/src/actions/init/getPostInitMessageDisplay.ts	100.0% (new)
packages/@sanity/cli/src/actions/init/readTemplateManifest.ts	100.0% (new)
packages/@sanity/cli/src/actions/init/types.ts	100.0% (new)
packages/@sanity/cli/src/commands/init.ts	91.5% (- 1.1%)

Comparing 5 changed files against main @ 133be82f2a246f5cc1573c8e2733d50a2b2ea746

Overall Coverage

Metric	Coverage
Statements	83.0% (±0%)
Branches	72.7% (+ 0.0%)
Functions	83.3% (+ 0.0%)
Lines	83.5% (±0%)

[mttdnt]

Saw claude mentioned that if a bad template manifest or the read fails we do not clean it up? Is it worth cleaning up here regardless or even in readTemplateManifest.ts?

[mttdnt]

Looks good! Just have one question and a change needed due to us changing zod to zod/mini

[mttdnt]

Will need an update to zod/mini per #847