ci/docs: add Foundry+Flutter pipeline, branching strategy, and PR checklist

[hooftly]

Summary

Introduces a CI pipeline for both contracts and mobile, fixes ABI/signature handling in the Flutter client, and extends contributor documentation with branching strategy and a PR checklist.

Changes

• CI
  • Added .github/workflows/ci.yml:
    • Contracts job: setup Foundry, forge build, forge test -vv
    • Mobile job: setup Flutter, flutter analyze, flutter test
• Mobile
  • mnemonic.dart: fixed BIP32 seed handling using Uint8List.fromList.
  • main.dart:
    • Switched to typed ABI encoding for execute(address,uint256,bytes).
    • Parsed ETH via EtherAmount.fromBase10String.
    • Constructed MsgSignature from signToUint8List.
    • Used typed ABI (TupleType) for getUserOpHash.
    • UI and status text formatting updates.
  • userop_signer.dart:
    • Left-padded r/s to 32 bytes in signature packing.
    • Adopted w3.MsgSignature; added hkdfIndex helper.
  • theme.dart: formatting and input border tweaks.
• Documentation
  • Added docs/dev/branching.md:
    • Defines Git Flow model (main, develop, feature/*, release/*).
    • Establishes Semantic Versioning with initial release v0.1.0.
    • Notes tagging strategy for releases.
  • Updated .github/PULL_REQUEST_TEMPLATE.md:
    • Added checklist for tests, docs, and lint checks.

Motivation

• Provide automated CI coverage for contracts and mobile codebases.
• Fix brittle signature/ABI logic in the mobile app for reliable AA flows.
• Establish consistent branching/release strategy and enforce quality standards with a contributor checklist.

Testing

• CI: Verified both workflows pass locally with equivalent commands:
  • forge build && forge test -vv
  • flutter analyze && flutter test
• Mobile: Confirmed test UserOp executes through bundler with correct encoding and signature.
• Docs: Verified new PR template renders correctly, and docs/dev/branching.md is included in the repo structure.