Docs: contributor onboarding commands incomplete/inconsistent across CONTRIBUTING.md and Copilot instructions

[codeforester]

Goal

Fix contributor-onboarding docs so the setup and validation commands they
give actually work as written, and so Copilot isn't given a different
validation command than every other contributor/agent.

Background

A documentation review of the process/CI/agent docs found several onboarding
commands that are incomplete or inconsistent with each other:

• CONTRIBUTING.md:76-97 ("Contributor Setup") only tells a new contributor
  to clone base-bash-libs, never base-cli — yet the pytest command given
  later in the same file (CONTRIBUTING.md:118-122) requires a base-cli
  sibling checkout (BASE_CLI_SOURCE_DIR=../base-cli/lib/python).
• CONTRIBUTING.md:127-132's base-test command for the "linked worktree"
  scenario omits BASE_CLI_SOURCE_DIR, but docs/testing.md:124-138
  explains that this exact scenario
  (~/work/base-worktrees/<slug>) breaks the sibling base-cli lookup and
  requires that variable — two of the three docs that give this command give
  an incomplete version.
• .github/copilot-instructions.md:39 tells Copilot to validate general
  changes with env -u BASE_HOME ./bin/base-test, while AGENTS.md:121 and
  CONTRIBUTING.md:142 instruct basectl test base for the same case —
  Copilot is given a different validation command than every other agent
  workflow described in the repo.

Scope

• CONTRIBUTING.md
• .github/copilot-instructions.md
• docs/testing.md (only if a cross-reference needs adding)

Acceptance Criteria

• CONTRIBUTING.md's Contributor Setup section either has the
  contributor clone base-cli alongside base-bash-libs, or clearly
  explains that it's only needed for the Python test command shown
  later, with a pointer at that point.
• Every copy of the linked-worktree base-test command in these docs
  includes BASE_CLI_SOURCE_DIR, matching docs/testing.md's
  explanation of why it's required.
• .github/copilot-instructions.md's validation command matches (or the
  doc explains why it intentionally differs from) AGENTS.md/
  CONTRIBUTING.md.

Validation

Follow CONTRIBUTING.md's Contributor Setup + Running Tests steps literally
on a clean checkout and confirm each given command succeeds without needing
an undocumented extra step.

Non-Goals

• Changing the actual test/setup tooling — this is about making the
  documented commands correct and consistent.

Project Metadata

• Priority: P1
• Size: M
• Area: Docs
• Initiative: Adoption Polish
• Agent Assignment: Copilot candidate after triage (mechanical command
  reconciliation, verifiable by literally running the documented steps)