Define and test the moving base-cli provider compatibility boundary

[codeforester]

Goal

Define and enforce the compatibility contract for every base-cli provider path so Base source validation cannot silently select a provider that fails Base adapters or documentation contracts.

Background

Base CI and the release BOM pin base-cli v0.4.3, and the pinned provider passes the Base suite. The documented source-checkout path in docs/testing.md tells contributors to clone the sibling base-cli checkout, and base_cli_runtime.sh prefers that moving checkout whenever it is present.

Against current base-cli origin/main, still reporting VERSION 0.4.3, the Base Python suite fails in two provider-boundary tests:

• The run-index adapter expects the older exact payload while base_cli._runtime.refresh_run_bundle_index emits additive complete and omitted_bundles metadata.
• The Base documentation test treats the provider new private Context._run_lease field as public because the exclusion list is hard-coded.

Base also imports private provider runtime state directly, while base_cli_adapters.provider only validates command-protocol symbols. This is a source-development and release-boundary failure, not a failure of the BOM-pinned v1.9.0 stack.

Scope

• Provider resolution and compatibility checks in lib/base/base_cli_runtime.sh and cli/python/base_cli_adapters
• Run-index and Context contracts used by Base
• Source-checkout documentation and tests for pinned, installed, explicit-source, and sibling-source providers

Acceptance Criteria

• The supported provider matrix is explicit, including whether a moving sibling checkout is supported, pinned, or only diagnostic.
• The run-index adapter has a documented payload contract and tests tolerate or intentionally reject provider-owned additive metadata with an actionable error.
• Private provider Context fields are excluded through a centralized contract or compatibility mechanism rather than a stale hard-coded list.
• Provider preflight checks the capabilities needed by every Base adapter that imports provider internals.
• CI and local source-checkout guidance exercise the same compatibility contract for the release-pinned and moving-source paths.
• A provider drift failure identifies the selected source and the repair path without weakening the released v1.9.0 pin.

Validation

• env -u BASE_HOME ./bin/base-test with the release-pinned provider
• The documented sibling source-checkout command with the current provider
• Focused run-index, context-documentation, and provider-preflight tests
• git diff --check

Non-Goals

• No unreviewed upgrade of the released base-cli pin.
• No change to the standalone base-cli project outside the compatibility contract needed by Base.

Project Fields

• Status: Backlog
• Priority: P2
• Area: Python
• Initiative: Contract Hardening
• Size: S
• Milestone: v1.10.0

Agent Assignment

• Human first; provider ownership and release compatibility need maintainer agreement before implementation.