chore(toolchain): restore lints/ + bump cargo-dylint binary to 6.0.1

Reverts the earlier deletion of the lints/ dylint crate per maintainer
feedback (@hzxa21 #25945 (comment)
+ @tygg direction). The original "upstream-blocked" framing was
inaccurate: dylint_linting / dylint_driver 6.0.1 do support rustc 1.98
via version-gated handling (#[rustversion::since(2026-03-18)] selects
self.env_depinfo on the new API). The actual gate-blocker was the
cargo-dylint *binary* baked into rw-build-env (ci/Dockerfile:83's
cargo install cargo-dylint@4.1.0 dylint-link@4.1.0), which was running
against rustc 1.98 internals and hit the removed ParseSess fields.

Restores everything that round 26 (15b57a0) removed:
- lints/ directory (15 files - crate, tests, ui, README, rust-toolchain)
- workspace.exclude entry lints in root Cargo.toml
- workspace.metadata.dylint section in root Cargo.toml
- ci/scripts/check-dylint.sh
- check-dylint Buildkite job in ci/workflows/pull-request.yml
- lints/ui/** entry in licenserc.toml exclude list
- comment ref to lints/README.md in ci/rust-toolchain

And actually fixes the dylint binary issue:
- ci/Dockerfile:83: cargo-dylint@4.1.0 -> 6.0.1, dylint-link@4.1.0 -> 6.0.1
- ci/.env: BUILD_ENV_VERSION v20260424 -> v20260614 (forces CI image rebuild
  so the new binaries take effect; the prior image cached 4.1.0)

Note: until the maintainer-side build-docker-image pipeline rebuilds
rw-build-env at v20260614, check-dylint will continue to fail on the
old 4.1.0 binary in the cached image. The next image rebuild fixes it.

Author: TennyZhuang

ci/.env

@@ -1 +1 @@
-BUILD_ENV_VERSION=v20260613
+BUILD_ENV_VERSION=v20260614