feat: add `basectl workspace test` to run tests across all managed workspace projects

[codeforester]

Problem

basectl test <project> works well for per-project test execution, but there is no workspace-level equivalent. Validating a cross-repo change (e.g., a shared library update affecting multiple downstream projects) requires developers to loop manually across repos or write a one-off shell script.

The workspace manifest already has complete knowledge of which repos are managed and what test commands they declare — this knowledge is currently unused at the workspace level.

Proposed solution

Add basectl workspace test [options] that:

1. Reads the workspace manifest to discover all managed projects.
2. For each project (or a filtered subset), runs the project's declared test command from base_manifest.yaml.
3. Emits per-project pass/fail results in the same structured format as workspace check --format json.

Proposed interface

basectl workspace test                          # run tests for all workspace projects
basectl workspace test --projects proj1,proj2  # run tests for specified projects only
basectl workspace test --fail-fast             # stop on first project failure
basectl workspace test --format json           # structured JSON output

Execution model

• Serial execution by default (consistent with docs/check-parallelism.md — predictable output, no interleaved logs).
• Each project's test command runs in the project directory with its environment activated (same as basectl test <project>).
• Summary at end: N passed, M failed, K skipped (no test command declared).

Acceptance criteria

• basectl workspace test is implemented as a new workspace subcommand.
• Runs declared test_commands for each managed project in the workspace manifest.
• --projects filter selects a subset of projects by name.
• --fail-fast stops after the first failing project.
• --format json emits structured output consistent with other workspace JSON outputs.
• Projects with no test_commands declared are skipped with a skip notice (not an error).
• Command is registered in docs/stability-tiers.md (Experimental initially).
• Help text and .ai-context/COMMANDS.md updated.
• Integration tests covering: all-pass, one-fail, fail-fast, --projects filter, skipped (no test command).