refactor(ampd): standardize env var names with AMP_prefix

[mitchhs12]

Standardizes ampd environment variable naming to use consistent AMP_* prefix for all CLI options.

Changes

• Renamed environment variables:

  • FLIGHT_SERVER → AMP_FLIGHT_SERVER
  • JSONL_SERVER → AMP_JSONL_SERVER
  • ADMIN_SERVER → AMP_ADMIN_SERVER

• Add deprecation migration at startup that:

  • Copies old env var values to new names
  • Emits warning to stderr for each deprecated var

• Update OpenCLI spec documentation

• Add upgrade guide entry for users

Note: Deprecation warnings use eprintln! because this runs before tracing is initialized (before clap parsing).

Backward Compatibility

• Old environment variable names continue to work but emit deprecation warnings. No breaking changes - users can migrate at their own pace.

Closes #1692

[mitchhs12]

Not sure whether the deprecation notice for this should be in the upgrading.md file.

How long should we keep this in here?

[LNSD]

You should update the feature docs for ampd too. To reflect the new env variable names.