[deploy-plugin DPB-2] Move pure deploy conventions with their constants to core

[rickylabs]

Part of #892 (RFC #891; corpus .llm/runs/plan-deploy-plugin--seed/ r5).

Move the activation/secrets/rollback/observability/health-gate conventions to plugin-deploy-core/conventions with their constants; runtime-overrides.ts and servy-config.ts stay with their bare-metal/leaf owners. Anti-scope: no convention behavior change (DP-2 §6, SF-2).

Acceptance

• gate: convention unit tests pass unchanged from the new home
• gate: adapters delegate (R-DEPLOY-3) — no forked convention copies remain

Metadata

Dependencies: DPB-1
Delivery shape: pure-module move