Remove lerna and replace its two uses with lighter alternatives:

- **`lerna version`** → `scripts/bump-version.js` — a ~80-line Node
script that updates all workspace `package.json` versions and
`@sentry-internal/*` cross-references to the exact new version. Modeled
after
[sentry-javascript#19325](getsentry/sentry-javascript#19325).
- **`lerna run build:tarball`** → `turbo run build:tarball` — Turborepo
is already used for all other cross-package tasks.

Lerna was pulling in a large transitive dependency tree including
packages with known vulnerabilities. This removes ~2,300 lines from
`yarn.lock`.

### Dependabot alerts resolved

**Fully resolved** (vulnerable package completely removed from
lockfile):

| Alert | Severity | Package | Summary |
|-------|----------|---------|---------|
| #217 | HIGH | `tar` | Symlink Path Traversal via Drive-Relative
Linkpath |
| #216 | HIGH | `tar` | Hardlink Path Traversal via Drive-Relative
Linkpath |
| #185 | HIGH | `tar` | Arbitrary File Read/Write via Hardlink Target
Escape Through Symlink Chain |
| #176 | HIGH | `tar` | Arbitrary File Creation/Overwrite via Hardlink
Path Traversal |
| #174 | HIGH | `tar` | Race Condition via Unicode Ligature Collisions
on macOS APFS |
| #172 | HIGH | `tar` | Arbitrary File Overwrite and Symlink Poisoning
via Insufficient Path Sanitization |
| #137 | HIGH | `axios` | SSRF and Credential Leakage via Absolute URL |

**Partially resolved** (some vulnerable entries removed, but the package
still exists via other dependency chains):

| Alert | Severity | Package | Remaining source |
|-------|----------|---------|-----------------|
| #213, #208, #207, #205 | HIGH | `minimatch` | `markdownlint-cli`,
`vite-plugin-dts` still pull in vulnerable versions |
| #161 | HIGH | `glob` | `markdownlint-cli`, `web-ext-run` still pull in
vulnerable versions |
| #136, #135 | CRITICAL | `form-data` | `jest` (via jsdom) still pulls
in `form-data@3.0.2` |
| #158, #157 | MEDIUM | `js-yaml` | `@changesets/cli`, `jest` still pull
in `js-yaml@3.14.1` |

The partially resolved alerts will be addressed in later phases (Phase
6: mop up remaining — bumping `markdownlint-cli`, `@changesets/cli`,
`jest`/rrvideo, etc.).

Part of the ongoing effort to resolve all Dependabot alerts on this
repo.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>