fix(core): migrate Gemini CLI contextFileName to context.fileName

[animeshxd]

Current Behavior

Gemini CLI v0.3.0 and higher (google-gemini/gemini-cli#7244 ) no longer supports the top-level "contextFileName" key in .gemini/settings.json. Workspaces using the setup-ai-agents generator or the Nx Gemini integration write and read contextFileName in the old flat format, which is silently ignored by newer Gemini versions, causing the context file to not be picked up at all.

Additionally, context.fileName can be a string[] in the new API, but the existing code only handled string, meaning multi-file configurations were not supported.

Expected Behavior

• The setup-ai-agents generator writes context configuration using the new nested format:

  "context": {
    "fileName": "AGENTS.md"
  }

• Any existing contextFileName key found in .gemini/settings.json is automatically migrated to context.fileName when the generator runs, without overwriting an already-set context.fileName.
• When context.fileName is a string[], the first entry is used as the primary file for writing agent rules.
• utils.ts resolves the context file path by preferring context.fileName over the legacy contextFileName, with full support for both string and string[] values.
• The root .gemini/settings.json is updated to use the new format.

Related Issue(s)

Fixes #

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