roadmap: ADRs, onboarding docs, versioned template upgrades

[tanbamboo]

Part of the P3 roadmap.

Three longevity items

• Architecture Decision Records — generate a docs/adr/ starter (ADR template + first record) so teams capture decisions alongside code.
• Contributor onboarding — a generated CONTRIBUTING.md aligned with the team-mode + PR workflow already described in AGENTS.md.
• Versioned template upgrades — an upgrade path so projects generated by an older CLI can pull in template improvements without a full re-scaffold.

Acceptance

• docs/adr/ scaffold generated on request.
• CONTRIBUTING.md matches the selected team mode.
• A documented upgrade command or migration guide exists.