scripts: replace buildkite-failures.ts with bk-backed ci:* helpers (#28877)

## What does this PR do?

Replaces `scripts/buildkite-failures.ts` (1078 lines of HTML scraping
against buildkite.com) with a thin `scripts/find-build.ts` that shells
to the [`bk`](https://github.com/buildkite/cli) BuildKite CLI.

- **`.bk.yaml`** — sets `selected_org: bun` / `pipelines: [bun]` so raw
`bk` commands work in this repo without `-p bun`
- **`scripts/find-build.ts`** — resolves current-branch / `#pr` / PR URL
/ branch / build# / buildkite URL → build number, then either prints it
(composable) or acts on it:
- `--status` — one-screen summary: build state, job counts, failed jobs,
failing test files so far (works on running builds)
- `--errors` — renders annotation `body_html` back to colored terminal
text, dedups near-identical per-platform output, tags each failure
`[new]` vs `[also on main]` by diffing against the last 5 finished main
builds
- `--logs` — saves full ANSI-stripped logs for every failed job to
`./tmp/ci-<build>/`
  - `--watch` — runs `bk build watch` on the resolved build
  - `--all` / `--no-compare` / `--no-dedup` modifiers
- **`package.json`** — `ci:find`, `ci:status`, `ci:errors`, `ci:logs`,
`ci:watch`. All accept the same target syntax.
- Docs in `CLAUDE.md`, `CONTRIBUTING.md`,
`docs/project/contributing.mdx`

Setup:

```sh
brew install buildkite/buildkite/bk
export BUILDKITE_API_TOKEN=<read-scoped token>
```

Notes:
- `bk build view -b <branch>` silently filters by `creator=<you>` and
returns the wrong build, so resolution uses `bk build list --branch`
instead.
- `bk`'s pipeline auto-detect misses because the git remote is `git@…`
while BuildKite stores `https://…`; `.bk.yaml` sidesteps that.
- Dedup only normalizes timing, CI-worker path prefixes, and whitespace
before comparing; error text, line numbers, and stack frames are
compared exactly. `--no-dedup` prints everything.

## How did you verify your code works?

Ran every mode against live builds: `ci:find main` / `-n 3`, `ci:find
'#28870'` (PR → branch → build via `gh`), `ci:status` on
running/passed/failed/canceled builds, `ci:errors` on build 43756
(renders annotations, dedups 19 platforms → 3 distinct bodies,
`[new]`/`[also on main]` tags), `ci:logs` on 43758 (122KB clean log
saved), `ci:watch` on a passed build (exits immediately with status).
Verified `$(bun run ci:find main)` composes cleanly (no ANSI in stdout)
with `bk build view`. oxlint and prettier pass.