bug: report the selected environment consistently in ConfigSnapshot

[codeforester]

Goal

report the selected environment consistently in ConfigSnapshot.

Background and evidence

Release-preparation review of 7d8e7d7e902ace88495c5e0ff3b9db4f4a7bf5d2 on macOS arm64. Python 3.13.15, Click 8.5.0, Typer 0.27.2.

BatteriesIncludedConfigLoader.load(..., environment="prod") loads environments/prod.yaml but returns snapshot.framework.environment == "dev" and provenance from the default/user environment. The public loader therefore exposes a contradictory snapshot. The App wrapper separately overrides ctx.environment, which hides this bug from the existing App-based environment test.

from pathlib import Path
from tempfile import TemporaryDirectory
from base_cli.config import BatteriesIncludedConfigLoader
with TemporaryDirectory() as d:
    user = Path(d)
    (user / "environments").mkdir()
    (user / "config.yaml").write_text("environment: dev\n")
    (user / "environments" / "prod.yaml").write_text("endpoint: production\n")
    s = BatteriesIncludedConfigLoader(user_config_dir=user).load(None, None, environment="prod")
    print(s.framework.environment, s.config, dict(s.provenance))

Actual: dev, {'endpoint': 'production'}, with environment provenance user. Expected: environment identity and provenance describe the explicit prod selection. The selector is validated and used for paths but never committed into merged before FrameworkConfig is constructed.

Scope and acceptance criteria

• Make an explicit environment selection authoritative in the returned framework snapshot and its provenance.
• Define and test whether environment files may change the selected environment; prevent a snapshot from claiming a different environment than the layer selected.
• Add direct public loader tests, including no base config, a conflicting user/project environment, and App parity.

Validation

Run tests/test_batteries_included_config.py, relevant lifecycle tests, and the full gate.

Source references:

• lib/python/base_cli/config.py
• lib/python/base_cli/_app_core.py
• tests/test_batteries_included_config.py

Non-goals

No release publication or unrelated API redesign. Preserve documented compatibility except for the defective behavior identified above.

Project fields

• Project: base-cli
• Status: Ready
• Priority: P2
• Area: Runtime
• Initiative: v1.0 Readiness
• Size: S
• Milestone: v1.0.0

Agent assignment

Assignee: @codeforester. Implementation may be handled through the normal issue-backed worktree and reviewed PR workflow.