Add environment variables to CommandOptions

[matt-edmondson]

Fixes #40.

Stacked on #43 (feat/39-working-directory), which introduces CommandOptions. Review that one first; this PR's diff is against it.

Problem

ProcessStartInfo.Environment was never touched, so the child always inherited the host's environment verbatim with no way to add, override or remove a variable for a single call. Environment variables are the only control surface some tools expose, so this blocked behaviour with no flag equivalent:

• GIT_TERMINAL_PROMPT=0 — without it a fetch/pull/push needing credentials blocks forever waiting on input that will never arrive, because output is redirected and no terminal is attached. The only mitigation left was a timeout, turning a clean "auth required" failure into an opaque hang-then-cancel.
• GIT_ASKPASS / SSH_ASKPASS — the supported way to supply credentials to a subprocess without putting them on a command line where any process listing can read them.
• LC_ALL=C — forces stable, machine-parseable output. Its absence is silent: parsing works on the developer's machine and fails only for users with a localized environment.

Approach

One new property on the CommandOptions record from #43 — no new overloads, which is what the options object was for:

public IReadOnlyDictionary<string, string?>? EnvironmentVariables { get; init; }

The entries are an overlay, not a replacement: a name not listed keeps whatever the caller had. A null value removes a variable, matching ProcessStartInfo.Environment semantics. Leaving the property null preserves today's behaviour exactly.

Elevation forces UseShellExecute, which has nowhere to carry an environment. Rather than let Process.Start fail with a message mentioning neither setting — or silently drop variables a caller may be relying on for credentials — combining the two throws ArgumentException up front.

Testing

• ExecuteAsyncShouldSetAnEnvironmentVariableForTheChildProcess
• ExecuteAsyncShouldOverrideAnInheritedEnvironmentVariable
• ExecuteAsyncShouldRemoveAnInheritedEnvironmentVariableWhenTheValueIsNull — asserts the inherited value did not reach the child, since an unset variable prints differently in cmd and sh
• ExecuteAsyncShouldInheritTheEnvironmentWhenNoVariablesAreGiven — pins that an unset overlay leaves previous behaviour untouched
• ExecuteAsyncShouldRejectEnvironmentVariablesCombinedWithElevation

Each test uses a variable name derived from its own method name, since the host environment is process-wide and tests run in parallel.

Full suite: 32 passed, 2 skipped (elevated tests self-skip to avoid a UAC prompt). Builds clean across all eight target frameworks — ProcessStartInfo.Environment is available on netstandard2.0, so no conditional compilation was needed.

🤖 Generated with Claude Code

https://claude.ai/code/session_01LwKTWcmxe5mh6DsLRj5NGQ

[sonarqubecloud]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud