[drauschenbach]

Summary

Adds a new lints/build-lints crate, which is a dylint based library for custom workspace rules.

Its first lint, io_try_without_context, rejects a ? that converts a std::io::Error into another error type without adding context. A bare I/O error says "No such file or directory (os error 2)" and nothing about which file or what was being done.

The first lint

io_try_without_context is the dylint project's sample lint. It also happens to be a good one, so it is included here to seed the pattern of where new lints go. It is a hard error by default. It flags x? when:

• x is a Result<_, std::io::Error>, and
• the enclosing function, closure or try block returns a different error type, so ? converts the error
  through From.

It doesn't flag ? from one std::io::Result into another. No conversion happens there, and the caller that converts away from std::io::Error is where the context goes. This also keeps std::io::Read and Write implementations easy to write.

Any conversion before the ? satisfies the lint: .map_err(...) into Error with Error::with_cause, anyhow's .context(...), or a crate's own conversion.

The crate

• Location: lints/build-lints/, outside both the cargo workspace and crates/. A dylint library builds on a
  nightly with rustc_private, while the workspace is pinned to stable 1.99. The build-xtask structural
  checks apply to every crate under crates/, and this crate couldn't satisfy them.
• Toolchain: pinned to nightly-2026-08-20, paired with the clippy_utils git rev in its Cargo.toml. The two
  change together (cargo dylint upgrade).
• Name: build-lints, not runvil-lints, because AGENTS.md defines every runvil-* crate as the Engine.
• Wiring: the root Cargo.toml lists the library under [workspace.metadata.dylint], so cargo dylint --all
  runs it from the repo root.
• More lints: register_lints registers each lint by hand, so the library can hold more than one; each lint
  is its own module with a UI test under ui/.

CI and docs

• CI: a new dylint job is added to ci-green. It installs cargo-dylint and dylint-link 6.3.2 and reads the
  nightly from lints/build-lints/rust-toolchain.toml, so the date lives only in that file. Because cargo
  dylint compiles the desktop app, it uses check-workshop-linux's setup: ubuntu-22.04, the Tauri packages,
  npm ci and the stub sidecar. It then runs the lint's UI tests and cargo dylint --all -- --locked
  --workspace --all-targets --all-features, and checks the tree is clean.
• AGENTS.md: the Verification section gets the custom-lints command, what to install first, and how to add
  a new lint.
• Plan: vibe/2026-11/2026-11-01-new-lints-crate.md.

Testing

• cargo test in lints/build-lints: the UI test passes. It covers four flagged cases (a custom From, the
  spelled-out Result, Box, a closure) and four allowed ones (map_err first, io to io, a non-io
  Result, Option).
• cargo dylint --all -- --locked --workspace --all-targets --all-features: no hits.
• Both clippy runs with CARGO_BUILD_WARNINGS=deny, cargo fmt --all --check, cargo test -p build-xtask and
  the docs-claims scan: all clean.
• nextest for runvil, build-xtask, build-ui and workshop-run-log: 340 passed.
• nextest for the Workshop crates: 313 passed, 1 failed.
  realtime_relay::overload::stalled_browser_cleanup_is_bounded_after_gateway_disconnect fails on every
  run, including with this PR's crate changes stashed, so it was already failing before this PR.
• The new dylint CI job hasn't run on GitHub yet; this PR will be its first run.