feat(config): accept ${env.X} in toolset env values (#2615)

[dgageot]

Toolset env values previously only understood the shell-style ${X} expansion syntax. When an agent config used the JS-template form ${env.X} — which already works in path fields like working_dir and path — the runtime would reject it with a hard "environment variable not set" error, silently breaking configs that mixed both styles. This is the first part of fixing the inconsistency tracked in #2615.

The fix extracts a shared NormalizeEnvRefs helper in pkg/path/expand.go that rewrites ${env.X} to ${X} before expansion, then calls it from pkg/environment/expand.go ahead of os.Expand. Richer JS expressions such as ${env.VAR || 'default'} are intentionally not evaluated in env values — only the plain ${env.X} alias is handled. The variable-expansion quick-reference table in docs/configuration/overview/index.md has been updated to reflect that env values now accept both forms.

A related cleanup drops the now-obsolete startup warning that flagged ${env.X} in toolset env values. The warning helper for shell and hook fields — which are forwarded verbatim to exec.Cmd with no expansion at all — was renamed warnPathField → warnExecField and its message corrected; the old message incorrectly told users to switch to ${X} or $X, neither of which is substituted in those fields.

[docker-agent]

Assessment: 🟢 APPROVE

The PR correctly implements NormalizeEnvRefs to rewrite ${env.X} → ${X} before os.Expand, and integrates it cleanly in both pkg/path/expand.go and pkg/environment/expand.go. The regex replacement string ${\} is valid Go regexp syntax and produces the expected output. The warning removal and warnPathField → warnExecField rename are consistent with the change. No bugs introduced by this PR were found.