[rush] Add an optional "description" field for projects in rush.json

[Xi Huang (yumikohey)]

Summary

rush.json is documented as a "centralized inventory of all projects and their important metadata", but a project entry has no place to record what the project is. Adding a free-form field today fails schema validation:

ERROR: JSON validation failed:
rush.json

Error: #/projects/0
       must NOT have additional properties: description

This PR adds an optional "description" string to project entries in rush.json, and exposes it through a new @beta RushConfigurationProject.description property so that tools and plugins can use it (for example, to show a one-line summary of each project in project listings or repo navigation tooling).

{
  "packageName": "my-app",
  "projectFolder": "apps/my-app",
  "description": "The main web application for the example repo"
}

Details

• rush.schema.json: add an optional description string to projects[] items. additionalProperties: false is unchanged, so misspelled fields are still reported.
• RushConfigurationProject: add description?: string to IRushConfigurationProjectJson, and a new @beta readonly description: string | undefined property populated from rush.json (undefined when omitted). The API report is updated accordingly.
• rush-init/rush.json: document the new field as a HYPOTHETICAL line in the demo project.

Rush does not interpret the value; it has no effect on install, build, change, or publish behavior. The change is purely additive, so existing rush.json files remain valid, and there is no meaningful performance impact.

An alternative is the "description" field in each project's package.json. Keeping a short summary in rush.json instead makes the central project inventory self-describing, so people and tools can understand the repository layout from one file without opening every package.json. This is especially useful in large monorepos.

How it was tested

• Added description to a project in the rush-npm.json test fixture, and extended RushConfiguration.test.ts to verify RushConfigurationProject.description for projects with and without the field.
• Ran rush build -t @microsoft/rush-lib (API report regenerated) and rush test -o @microsoft/rush-lib: all tests pass.
• Validated the expanded rush-init/rush.json template against the old and new schemas: the old schema rejects #/projects/0 with the error above, and the new schema accepts it.
• Manually validated a large monorepo's rush.json in which every project entry has a description: it fails validation with the current schema and passes with this change, and RushConfiguration.loadFromConfigurationFile() returns each project's description.

Impacted documentation

• https://rushjs.io/pages/configs/rush_json/ (generated from the rush-init/rush.json template, which this PR updates)

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