ReviewAgent: make helper configuration inputs explicit #2408

Follow-up to #2402, as requested in review: #2402 (comment)

Several review-engine helpers depend on variables inherited through PowerShell dynamic scope. For example, Invoke-CopilotCli accepts only Prompt while relying on caller configuration for the executable, model, working directory, report path, and execution settings. Assert-Config has similar implicit inputs. This makes isolated testing and future refactoring fragile.

Scope:

• Replace implicit configuration inputs with explicit, typed parameters or a narrowly scoped configuration/context object, starting with Invoke-CopilotCli and Assert-Config.
• Update direct callers and audit related root/leaf execution helpers for the same pattern without introducing global state.
• Preserve root/leaf model selection, exact report-file write permissions, child-process environment isolation, npm launch behavior, and existing failure/telemetry semantics.
• Add focused tests that call helpers without pre-populated caller-scope variables, alongside existing root/leaf execution tests.

This is intentionally tracked separately from the correctness fixes in the PR to avoid a broad late-stage refactor.