docs: address review feedback on env-var examples

- Linux/macOS: actually set GITHUB_PAT instead of inlining via subshell, matching the heading.
- PowerShell: use Select-Object -First 1, split with max 2 parts, and trim quotes/whitespace so common .env formats work.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Copilot

docs/installation-guides/install-claude.md

@@ -39,12 +39,14 @@ claude mcp add-json github '{"type":"http","url":"https://api.githubcopilot.com/
 
 With an environment variable (Linux/macOS):
 ```bash
-claude mcp add-json github '{"type":"http","url":"https://api.githubcopilot.com/mcp","headers":{"Authorization":"Bearer '"$(grep GITHUB_PAT .env | cut -d '=' -f2)"'"}}'
+export GITHUB_PAT="$(grep '^GITHUB_PAT=' .env | cut -d '=' -f2-)"
+claude mcp add-json github '{"type":"http","url":"https://api.githubcopilot.com/mcp","headers":{"Authorization":"Bearer '"$GITHUB_PAT"'"}}'
 ```
 
 With an environment variable (Windows PowerShell):
 ```powershell
-$env:GITHUB_PAT = (Get-Content .env | Select-String "^GITHUB_PAT=").ToString().Split("=")[1]
+$githubPatLine = Get-Content .env | Select-String "^\s*GITHUB_PAT\s*=" | Select-Object -First 1
+$env:GITHUB_PAT = ($githubPatLine.Line -split "=", 2)[1].Trim().Trim('"').Trim("'")
 claude mcp add-json github "{`"type`":`"http`",`"url`":`"https://api.githubcopilot.com/mcp`",`"headers`":{`"Authorization`":`"Bearer $env:GITHUB_PAT`"}}"
 ```