refactor: add environment accessor for Zig 0.16 prep

[forketyfork]

Issue

Prepare the environment-access and allocator portions of the staged Zig 0.16 migration (Task 3).

Solution

• Add and register src/env.zig with the planned process-environment accessor and tests.
• Route all 23 getenv call sites across 11 files through env.get, including aliased posix.getenv calls in shell.zig and logging.zig.
• Rename both GeneralPurposeAllocator instances to DebugAllocator.
• Add the minimal build-graph alias required for the existing standalone MCP control module to resolve the plan-prescribed relative environment import.
• Check off only the completed Step 3 items in the migration plan.

Context

This is the Zig 0.15.2-compatible preparation commit for the later Zig 0.16 toolchain flip. The environment accessor retains the exact std.posix.getenv return type. The MCP build-graph alias is necessary because Task 2 compiles src/app/control.zig as a separate module whose root otherwise rejects ../env.zig as outside its module path.

Test plan

• zig build — passed locally with directly installed Zig 0.15.2.
• zig build test — passed locally, run unpiped for the exit-code verdict.
• just lint — passed locally; test registry reports all 45 test files registered.
• zig fmt --check src/ and ./scripts/check-test-registry.sh — passed.
• The plan Step 3 manual check (run the app and exercise config loading, recent folders, and ~ path abbreviation) could not be performed in this Linux sandbox because it requires the SDL3/macOS UI. A human on macOS or CI with an appropriate runtime must perform/cover it; it is not being claimed as completed.