Prepare uni-7 v31 upgrade instructions

[juno-ai-dev]

Goal

Prepare the smallest credible uni-7 v31 upgrade-instructions change, following the merged v30 precedent in CosmosContracts#388.

Expected diff

• add uni-7/02_V31_UPGRADE.md
• update uni-7/README.md to point at the current upgrade

Verified inputs

• candidate source: juno-ai-dev/juno:v31
• upstream integration PR: feat(v31): integrated release candidate CosmosContracts/juno#1223
• upgrade plan name: v31
• v31 store upgrades: none
• current uni-7 version: v30.0.0

Release gates before upstream submission

• feat(v31): integrated release candidate CosmosContracts/juno#1223 is green and integrated
• immutable v31.0.0 release tag and commit exist
• exact source build succeeds with the pinned Go toolchain
• junod version --long output and wasmvm linkage are verified
• upgrade height and UTC estimate are selected from fresh uni-7 block-time data
• release assets/provenance are inspected; operator path matches what was actually published
• live RPC/API endpoints and all post-upgrade commands are verified
• final two-file diff is reviewed and read back from GitHub

Non-goals

• no governance proposal payload
• no speculative release SHA, checksum, height, or date
• no upstream PR until the release inputs and schedule are fixed