Replace GCP_PROJECT with GOOGLE_CLOUD_PROJECT

[Copilot]

Aligns environment variable naming with GCP's standard convention. Breaking change acceptable for pre-1.0 project.

Changes

• internal/config/config.go: Read GOOGLE_CLOUD_PROJECT instead of GCP_PROJECT
• main.go: Update error messages to reference new variable name
• README.md: Update configuration documentation

- ProjectID: getEnv("GCP_PROJECT", ""),
+ ProjectID: getEnv("GOOGLE_CLOUD_PROJECT", ""),

- log.Fatal("GCP_PROJECT environment variable must be set")
+ log.Fatal("GOOGLE_CLOUD_PROJECT environment variable must be set")

Original prompt

This project uses the "GCP_PROJECT" envirionment variable. This doesn't match the standard GCP uses which is "GOOGLE_CLOUD_PROJECT". Update the project to use "GOOGLE_CLOUD_PROJECT" instead. This is allowed to be a breaking change as this proejct isn't 1.0 yet. Just update all references everywhere so it uses the correct envirionment variable.

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (df9922b) to head (16301ac).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
main.go	0.00%	2 Missing ⚠️
internal/config/config.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #113   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files          3       3           
  Lines        120     120           
=====================================
  Misses       120     120           

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