fix(opencode): adopt legacy artifact receipts safely

[pacphi]

Closes #92.

What changed

• adopts exact marker-bearing OpenCode plugin, agent, stamp, and skill artifacts when legacy ownership receipts are absent
• preserves edited, stale, foreign, or non-null mismatched receipt state without laundering ownership on a later pass
• fails closed on malformed receipt containers and leaves all artifact slots untouched, including absent directories
• surfaces read-only adoption and malformed-ledger status with actionable remediation
• keeps receipt-only adoption idempotent while reporting marker persistence accurately

Safety contract

Adoption occurs only when the receipt is absent/null, the artifact carries the agentic-kit marker where applicable, and its bytes exactly equal the current desired bytes. The marker-free agent stamp is adopted only when its canonical bytes and every independently accepted agent file match.

Verification

• pnpm run check
• pnpm audit --audit-level high
• pnpm run test:surface
• pnpm run lint:links:internal
• focused OpenCode/status suite: 75/75 passing
• independent reconciliation review: no blockers