--test-command can verify against a richer environment than CI, producing a false green

[lorenzoliuzzo]

Measured on a real run against an out-of-fleet target.

my-coder reported the run as fully successful:

{"outcome": "success", "detail": "opened draft PR #10", "issue": 2,
 "files_touched": ["main.py", "tests/test_dataset.py"],
 "tests_passed": true, "cost_usd": 0.77}

The target repo's CI then failed on that exact commit:

ImportError while importing test module tests/test_dataset.py
    from torchsummary import summary
E   ModuleNotFoundError: No module named 'torchsummary'

The session wrote a test importing a module whose dependency the target repo had never declared. --test-command had been pointed at the operator's existing local .venv (per #15/#20, so the session would not spend its wall clock on a cold install), and that venv happened to have the package installed from earlier ad-hoc work. So _tests_pass returned True against an environment strictly richer than the declared one.

tests_passed: true currently means "green in whatever environment the operator supplied", but it is consumed — by the PR readiness checklist and by anyone reading the ledger — as "green". Those differ exactly when the target's declared dependencies are incomplete, which is precisely the bug class a fresh-environment run would catch.

This is the direct trade-off against #20: the cheap path (reuse a warm environment) and the trustworthy path (build from declared dependencies) are opposed, and right now the tool takes the cheap path while reporting the trustworthy one's verdict.

Acceptance criteria

• tests_passed distinguishes "green in a supplied environment" from "green in one built from the target's declared dependencies"; the ledger and the PR body carry which one it was.
• When a supplied --test-command is used, the PR body says so explicitly rather than implying an unqualified green.
• Ideally the declared-dependency environment is resolved from a warm cache, so the trustworthy path is also the cheap one (see Building the target environment from scratch consumes the whole wall clock #20) rather than a choice between them.