Fix Gemini CLI extension issues (gm-gc)

[lanmower]

This PR fixes several issues identified during testing with the Gemini CLI (gm-gc platform):

1. Agent Validation: Gemini CLI was rejecting agents with agent: and enforce: keys in their frontmatter. The transformAgentFrontmatter function in platforms/cli-config-shared.js was updated to correctly strip these keys even with leading whitespace.
2. Context Loading: Switched the primary context file from GEMINI.md to AGENTS.md for the Gemini platform. This ensures that the core project rules are loaded correctly by Gemini CLI's memory system.
3. Missing Files: Ensured AGENTS.md is included in the package manifest and handled correctly by the installer scripts.
4. Hook Execution: Gemini CLI treats non-zero exit codes from hooks as failures, which caused deadlocks when a hook correctly wanted to "block" an action (exit code 2). Updated gm-starter/bin/plugkit.js to map exit code 2 to 0 for hooks, allowing the CLI to process the JSON decision instead of crashing.
5. Tool Mappings: Updated platforms/cli-config-factory.js to include correct tool names for skill and agent on the Gemini platform.
6. Build Reporting: Updated lib/build-reporter.js regex to recognize ${extensionPath} as a valid path variable, removing false-positive warnings during the build process.

Verified the fixes by building the platforms and running gemini in a test directory with the generated extension.

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PR created automatically by Jules for task 1162609706878532573 started by @lanmower

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