Document GLAB_NO_PROMPT and clarify NO_PROMPT deprecation
## Summary
The `GLAB_NO_PROMPT` environment variable is not discoverable. It is referenced only in `cmd/glab/main.go` and appears in no user-facing documentation, help text, or generated docs. The only documented form is the legacy unprefixed `NO_PROMPT`, which is actually deprecated.
## Background
Prompt disabling is resolved in `cmd/glab/main.go` with the precedence `NO_PROMPT` > `GLAB_NO_PROMPT` > the `no_prompt` config key, all funneled through `IOStreams.SetPrompt`. Both env vars go through `utils.IsEnvVarEnabled`, which prints a deprecation warning for any non-`GLAB_`-prefixed variable (steering users to `GLAB_NO_PROMPT`). Per the README, all glab environment variables are prefixed with `GLAB_` in version 2.0.0 and later, so `GLAB_NO_PROMPT` is the current/preferred name and `NO_PROMPT` is the deprecated alias.
## Problems
1. `GLAB_NO_PROMPT` is undocumented everywhere (README, generated docs, help text).
2. `NO_PROMPT` is presented as the primary variable in the docs, when it is in fact deprecated.
3. The accepted values are not documented. `SetPrompt` accepts `true`/`1` (disable) and `false`/`0` (enable).
4. The `IsInteractive` doc comment in `internal/iostreams/iostreams.go` is stale, referencing only `NO_PROMPT`.
## Proposed changes
- Update the `help:environment` annotation in `internal/commands/root.go` to make `GLAB_NO_PROMPT` primary (documenting accepted values) and `NO_PROMPT` a deprecated alias, then regenerate `docs/source/_index.md` with `make gen-docs`.
- Update the README environment variable table: add a `GLAB_NO_PROMPT` row and mark `NO_PROMPT` as deprecated.
- Fix the stale `IsInteractive` doc comment in `internal/iostreams/iostreams.go`.
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