docs: clarify the installer already inits the current project (#826)

[Dashsoap]

Fixes #826.

What

The Quick Start lists codegraph init as step 3 as if it were always required after install. This clarifies that the interactive installer already initializes the project it is run in, so step 3 only applies to other projects.

Why

Step 1 of the same Quick Start already documents that the installer will:

• Initialize your current project (local installs only)

This is real — src/installer/index.ts runs initializeLocalProject() on a local install, which calls CodeGraph.init() + indexAll():

// Step 6: for local install, initialize the project.
await initializeLocalProject(clack, useDefaults);

So a user who follows the steps literally runs codegraph init (step 3) in the same project the installer just initialized, and gets:

Already initialized in /path/to/project
Use "codegraph index" to re-index or "codegraph sync" to update

That is exactly what the reporter hit in #826. This is a docs-only clarification:

 ### 3. Initialize Projects
+
+The interactive installer in step 1 already initializes **the project you ran it in**
+(it runs `codegraph init` for you on a local install ...). For that project you can
+skip straight to step 2.
+
+To index **other** projects, run `codegraph init` in each one:

 cd your-project
 codegraph init

No code change.