chore: clippy is disabled and has a 61-site backlog blocking `-D warnings`

[sanity]

Problem

cargo clippy --workspace --all-targets -- -D warnings does not pass, and the clippy workflow is checked in as .github/workflows/clippy.yml.disabled, so nothing catches new lints. Found while verifying #572, where the only way to get clippy coverage over the changed code was to allow the lints that stop it early.

Measured on main @ 9e663e3 with rustc/clippy 1.94.1 (the rust-toolchain.toml pin):

It stops at 7 errors in river-core, so ui/ and cli/ are never linted at all:

lint	count	site
clippy::doc_lazy_continuation	5	common/src/room_state/ban.rs:41-45
clippy::assertions_on_constants	1	common/src/room_state/dm_body.rs:358
clippy::repeat_once	1	common/src/room_state/message.rs:2116

Allowing those three lets it finish, and it finds 54 more across 17 files:

 12  ui/src/room_data.rs
  6  ui/src/components/conversation.rs
  5  ui/src/components/app/chat_delegate.rs
  5  ui/src/components/app/notifications.rs
  5  ui/src/util/confusable.rs
  4  ui/src/components/app/freenet_api/response_handler/get_response.rs
  3  ui/src/components/room_list/receive_invitation_modal.rs
  2  common/tests/dm_global_cap_test.rs
  2  cli/src/commands/debug.rs
  2  cli/src/version_check.rs
  2  ui/src/nickname.rs
  1  cli/src/api.rs
  1  ui/src/components/room_list/dm_rail_section.rs
  1  ui/src/components/app/freenet_api/room_synchronizer.rs
  1  ui/src/signing.rs
  1  ui/src/util/signal_guard.rs
  1  ui/src/util.rs

Reproduce:

cargo clippy --workspace --all-targets -- -D warnings
cargo clippy --workspace --all-targets -- -D warnings \
  -A clippy::doc_lazy_continuation -A clippy::assertions_on_constants -A clippy::repeat_once

Why it matters

The backlog is the reason the workflow is disabled, and the disabled workflow is the reason the backlog grows — so it does not resolve on its own. The concrete cost today is that a PR touching ui/ or cli/ gets no clippy signal on its own changes without hand-allowing unrelated lints first, which is easy to skip and easy to get wrong.

None of these look like real bugs on inspection — they are style and doc-formatting lints (doc_lazy_continuation, useless_vec, needless_borrows_for_generic_args, unused_mut, nonminimal_bool, repeat_once). That is what makes them safe to clear in one pass, and it is also why they should not ride along inside unrelated feature/fix PRs (AGENTS.md: one logical change per PR).

Suggested shape

1. One chore(lint): PR clearing all 61 sites, no behaviour change.
2. Re-enable clippy.yml in the same PR so the count cannot regress.

Not urgent, but it should be someone's deliberate task rather than a thing every PR author rediscovers.

[AI-assisted - Claude]