Upgrade Slither CI to 0.11.6

[mswilkison]

Closes #187.

Upgrade CI from Slither 0.8.0 to 0.11.6, with crytic-compile 0.4.2 and solc-select 1.2.0 pinned in requirements-slither.txt. The job uses Python 3.13, checks Python dependency consistency, and logs the resolved toolchain. The README includes local setup and an upgrade procedure. The documented .venv-slither/ is excluded from both Git and Prettier, so formatting commands skip installed Python dependencies.

This PR is stacked on #182 (chore/dependency-toolchain-refresh, base 14843ac). Merge #182 first, then retarget this PR to main. It uses that PR's Node 22 / Hardhat 2 / Ethers 5 toolchain.

Slither completes with the OpenZeppelin upgrades plugin enabled, resolving the historical crash that blocked #96. All Solidity edits are comments: remove 13 obsolete suppressions and document four narrowly scoped suppressions. The internal SafeTUpgradeable.safeTransferFrom adapter's two callers pass msg.sender; the three unindexed-event notices retain the existing event ABI and log encoding. Existing detector/path exclusions and the default failure policy are preserved.

Validation:

• Fresh yarn install --frozen-lockfile and python -m pip check pass.
• slither . performs a clean compilation and finishes with 0 findings across 84 contracts and 95 detectors.
• yarn build, yarn test (380 passing), and yarn format (including type checking) pass.
• All 25 repository-owned deployable artifacts retain identical ABIs, link references, and creation/runtime bytecode excluding compiler metadata. Solidity remains at 0.8.9.
• Isolated gate checks confirm an unsuppressed informational finding exits 255 and invalid Solidity exits 1.
• actionlint 1.7.12 and git diff --check pass.
• The optional unused-ignore audit now reports only the existing dependency directive in node_modules/@thesis/solidity-contracts/contracts/token/ERC20WithPermit.sol (reported twice); repository-owned directives are clean.

Local runtime: Python 3.13.7, Node 22.23.1, Yarn 1.22.22. CI used Python 3.13.15 and confirmed 0 Slither findings, compilation of 75 Solidity files, and 380 passing tests.

GitHub Actions passed on 20a20d5: Solidity build/tests, Slither, and deployment dry run, formatting and type checks, and documentation preview.