refactor(ampd): standardize env var names with AMP_prefix

[mitchhs12]

Standardize environment variable names under the AMP_ namespace and rewrite subcommand help text with precise behavioral descriptions.

• Rename FLIGHT_SERVER, JSONL_SERVER, ADMIN_SERVER to AMP_FLIGHT_SERVER, AMP_JSONL_SERVER, AMP_ADMIN_SERVER in solo/server subcommands
• Remove GM_* env var bindings from ampctl generate (CLI-only flags)
• Rewrite clap doc comments with port numbers, behavioral notes, and endpoint-selection semantics
• Update feature docs with configuration tables, env var references, and status frontmatter

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Note

Low Risk
Primarily CLI/env-var surface-area and documentation changes; runtime behavior is largely preserved via backward-compatible env var handling, with limited risk of misconfiguration during the transition.

Overview
Standardizes ampd CLI environment variables for endpoint selection by renaming FLIGHT_SERVER/JSONL_SERVER/ADMIN_SERVER to AMP_FLIGHT_SERVER/AMP_JSONL_SERVER/AMP_ADMIN_SERVER, while adding backward-compat checks that honor the old names with deprecation warnings.

Removes all GM_* environment-variable bindings from ampctl manifest generate, making those parameters strictly CLI flags. Updates CLI help text and feature documentation to reflect the new env var names, clarify endpoint-selection behavior (including ports), and adds/marks feature docs as stable (including a new app-ampd-migrate doc).

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[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]

@mitchhs12 I am taking over this PR

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