docs: reserve good first issues for contributor-led work

[codeforester]

Goal

Clarify in CONTRIBUTING.md that issues labeled good first issue are reserved for contributor-led work and are excluded from maintainer implementation trains by default.

Background

The README links newcomers to the open good first issue queue. Issues #503 and #504 were explicitly labeled for first-time contributors, but were pulled into a maintainer PR train because they were in Ready status. This exposed ambiguity: Project readiness does not indicate that maintainers should take ownership of contributor-reserved work. Both issues currently have active PRs (#513 and #512).

This also supports the contributor funnel established by #499.

Scope

• Amend CONTRIBUTING.md to define good first issue as work intentionally available to community contributors.
• State that maintainer sweeps and implementation trains must exclude issues carrying this label by default, regardless of Project status, and report them separately as contributor-reserved.
• Explain that if maintainers explicitly take over a labeled issue, they should remove the label once it is no longer available for contributors and follow the normal issue/PR workflow.

Acceptance Criteria

• CONTRIBUTING.md clearly distinguishes Project readiness from maintainer ownership.
• The maintainer issue-triage guidance explicitly excludes good first issue items from implementation trains unless they are specifically selected.
• The guidance explains when to remove the label if maintainers take over an issue.
• No library code, existing issue labels, or replacement good-first issues are changed or created by this documentation PR.

Validation

• Run ./tests/validate.sh.
• Run tests/lint-warnings.sh.
• Confirm the contributor workflow wording agrees with the README good-first-issues link and current Project status conventions.

Non-Goals

• Do not remove the good first issue label from tests: cover leading empty fields in base_str_split #503 or tests: cover multi-value prepend order in base_list_prepend #504 in this PR. Do that after this policy change is merged.
• Do not create replacement good-first issues here. Create two new contributor issues only after the outstanding issue work is complete, as a separate follow-up.

Project Fields

• Status: Backlog
• Priority: P2
• Area: Product
• Initiative: Adoption Polish
• Size: S
• Milestone: v2.2.0
• Target date: unset.

Agent Assignment

• Assignee: Unassigned.
• This is a small, focused documentation task.