chore(studio): keep useTrackExperimentExposure across experiment gaps (#49765)

## Summary

Adds a narrow knip suppression so
`hooks/misc/useTrackExperimentExposure.ts` isn't flagged as an unused
file when no experiment is currently consuming it.

## Why

Different from the other files #49719 cleaned up. Those were one-off
feature code (specific banners, mutations, table rows) — genuinely dead
when their feature was removed. This hook is **shared experiment
plumbing**: every A/B experiment plugs into it, and we run experiments
frequently. Gaps between experiments are normal.

Deleting and re-adding it each cycle:
- Churns the codebase for no real cleanup value
- Adds review surface to every next experiment PR (has to also add the
hook back)
- Caused a stacked-merge race today — #49719 deleted it, #49534
reintroduced a usage in `plan-presentation.ts`. Both PRs' CI ran green
on their own bases, but merging both broke master's typecheck + Studio
deploy. Fixed in #49763.

## What this changes

One entry in `knip.jsonc` under
`workspaces["apps/studio"].ignoreIssues`:

\`\`\`jsonc
"hooks/misc/useTrackExperimentExposure.ts": ["files"]
\`\`\`

`ignoreIssues` (not `ignore`) is the right knob per the config's own
guidance — it suppresses only the "unused file" issue for this specific
path, while knip still traces the file's imports so anything it depends
on stays honestly tracked.

## Test plan

- [ ] `pnpm knip --workspace apps/studio` still clean
- [ ] With no experiment referencing the hook, knip does not report it

knip.jsonc

@@ -72,6 +72,7 @@
         // read are documentation of the valid values, not dead code. Scoped to
         // this file so `enumMembers` keeps working everywhere else.
         "data/database/constraints-query.ts": ["enumMembers"],
+        "hooks/misc/useTrackExperimentExposure.ts": ["files"],
       },
       // `vercel` is a globally installed CLI used by the `deploy:staging` script
       "ignoreBinaries": ["vercel"],