Define a co-maintainer onboarding and bus-factor mitigation plan

[codeforester]

Goal

Give prospective adopters a documented answer to "what happens if the sole maintainer becomes unavailable," and take a concrete first step toward reducing that risk.

Background

Base is authored solely by one maintainer across its full commit history. This is repeatedly the top objection raised during due diligence by teams considering Base for org-wide rollout: it is infrastructure meant to be wired into shell startup and CI across many repositories, so its continuity risk is different in kind from a library dependency that can simply be pinned. docs/adopter-readiness.md-style candor elsewhere in the docs set makes this a natural next gap to close explicitly rather than leave implicit.

Scope

• Publish a maintainer-continuity statement (in SECURITY.md or a new docs/governance.md) describing current bus-factor status plainly.
• Define what "maintainer unresponsive" looks like operationally (e.g., no merged commits or triage for N weeks) and what recourse adopters have (fork rights already covered by Apache-2.0, but also: who else has push access, is there a designated backup reviewer).
• Identify and document a path to onboard at least one co-maintainer or trusted reviewer, even in a limited capacity (e.g., release sign-off, security triage).
• Cross-link this from docs/product-assessment.md under adoption risk.

Acceptance Criteria

• A governance/continuity doc exists and is linked from README.md.
• At least one concrete next step toward a second maintainer or backup reviewer is identified, even if not yet filled.
• docs/product-assessment.md reflects the mitigation rather than only naming the risk.