Add btt test-tree checking

[Maddiaa0]

Adds btt, which checks each test file against a .tree spec that sits next to it. This PR only sets the tool up and adds one example. Trees for the rest of the suite will come in later PRs.

What's included

• btt.toml, with the vendored Solidity pack in .btt/packs/solidity. btt only ships Rust and TypeScript packs; this one comes from the Solidity example in btt's README, with two changes: test names use a test prefix to match this repo, and scaffolded bodies use revert("unimplemented") because the forge-std version here has no vm.skip.
• A worked example: test/auth/Owned.tree. The four Owned tests are renamed to match its leaves, and their .gas-snapshot entries are renamed with the gas values unchanged.
• A btt CI job that installs btt-cli 0.2.0 from crates.io with --locked and runs btt check test.
• A short "Test trees (btt)" section in the README, and the Claude skill that btt init --skill generates, at .claude/skills/btt/SKILL.md.

How the trees map to tests

• Foo.tree sits next to Foo.t.sol, and its first line is the test contract name.
• Each it … leaf is one test: test + the leaf text in PascalCase, e.g. "it lets the owner set a new owner" → testLetsTheOwnerSetANewOwner.
• Trees are flat, because Solidity can't nest contracts. Put conditions in the leaf text instead of when/given branches.

CI behaviour

Test files without a tree (44 of them for now) are warnings, not errors, so the job passes today. Once every file has a tree, uncovered in btt.toml can be changed to error.

Run it as btt check test, not plain btt check. There's no exclude option, and plain btt check also scans the .t.sol files inside the lib/ submodules.

Testing

• btt check test: 1 tree file, 44 uncovered, 0 errors.
• forge test: 580/580 pass.

🤖 Generated with Claude Code