EVM proposals support specifications

[MCarlomagno]

Feature spec for adding EVM proposal sharing and signing support to Guardian.

It turns the previous draft into an implementable v1 contract by defining the EVM account model, request auth, proposal payload shape, proposal identity, signer semantics, and rollout approach. The feature remains scoped to account configuration plus proposal create/list/get/sign. Execution, delta/state flows, and reconciliation remain out of scope.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.46%. Comparing base (d80b017) to head (1e6bb9f).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #184      +/-   ##
==========================================
- Coverage   73.50%   67.46%   -6.04%     
==========================================
  Files          81       99      +18     
  Lines       14246    16045    +1799     
==========================================
+ Hits        10471    10825     +354     
- Misses       3775     5220    +1445     

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