docs(contracts): document contract immutability as a deliberate security property

[olu-cmd]

Summary

• Documents contract immutability in docs/contracts.md as a deliberate security property, not an omission: no vault or adapter contract exposes an update_current_contract_wasm entry point.
• Explains why an admin-gated upgrade() was considered and rejected, and how redeployment friction is already handled by existing tooling (deploy-testnet.sh for redeployment, migrate_adapter for moving live positions without a manual withdraw-then-deposit cycle per user).

Closes #574

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[collinsezedike]

@olu-cmd I need a few changes before this can merge:

• The title (feat: doc added) doesn't match CONTRIBUTING.md's Commit Convention: it becomes the squash commit message, so it needs the right type (docs: for a docs-only change) and an imperative subject ("document" not "added"). Read the Commit Convention section for the exact format.
• The PR body needs to follow CONTRIBUTING.md's required template (## Summary / ## Test plan), not just Close #574 and a one-line note.
• docs/contracts.md:11 uses em dashes ("None of Meridian's contracts — vault or adapters — expose an"). Please rewrite as plain sentences without them.

The content itself is accurate and well-placed, this is about form, not substance.

[collinsezedike]

Verified both factual claims directly against the contract source: no adapter or vault exposes an upgrade entry point, and migrate_adapter genuinely moves the whole vault-wide position atomically with a slippage bound, matching what's documented. Fixed the PR title (feat: doc added, wrong type and non-descriptive, and squash merge makes it the permanent commit message) to docs(contracts): document contract immutability as a deliberate security property. Merging now.