feat(push): expand default ignore list and exempt features/

[QT-7274]

Why

acai push --all currently fails with Invalid string length on large repos that contain build artifacts under directories that aren't in the default ignore list (e.g. Rust target/, Tauri src-tauri/target/, Java/Gradle build/, Next.js .next/, etc.). The CLI scans every file looking for ACID references, and a 10 GB build directory can push the resulting JSON payload past V8's max string length (~512 MB).

I noticed this on a Tauri project that has ~10 GB under src-tauri/target/ and ~840 MB under crates/. Reproduction is simple: any reasonably large compiled project will trigger it.

What

1. Extend IGNORED_REF_DIRS to cover the most common build/cache/IDE directories:

   • Build outputs: target, build, out, bin, obj, .next, .nuxt, .svelte-kit, .turbo, .cache, .parcel-cache, .vite, .astro, DerivedData, Pods
   • Python: __pycache__, .venv, venv, .tox, .mypy_cache, .pytest_cache, .ruff_cache
   • Go/PHP: vendor
   • IDE/OS: .idea, .vscode, .DS_Store

2. Intentionally do NOT add src-tauri. It contains real Rust source under src-tauri/src/** that should be scanned. The heavy artifact is src-tauri/target/, which is already covered by the new target entry. (This was raised in adversarial review and corrected before submitting.)

3. *.egg-info-style globs are intentionally NOT added. The set uses Set.has(name) exact-name matching, so glob entries silently no-op. Glob support is introduced in a follow-up PR (`.acaiignore`).

4. Exempt the canonical features/ tree. A user can have a feature spec at `features/build/login.feature.yaml` (where `build` is the product name). Without an exemption, the new `build` entry would skip the entire subtree, silently dropping their spec. `walkFiles` now bypasses `IGNORED_REF_DIRS` checks once the path is under `features/`.

Tests

4 new cases added to `src/core/push.test.ts`:

• `scanPushRepo skips extended build-output directories like target/, build/, .next/` — verifies the extended list is wired up.
• `scanPushRepo descends into src-tauri/src so Rust source ACIDs are discovered, while src-tauri/target is still skipped via the target rule` — pins the explicit decision not to add `src-tauri`.
• `scanPushRepo descends into features// even when matches a build-output directory (e.g. features/build/)` — covers the exemption.
• (existing) `scanPushRepo respects feature-name filters for both specs and refs` — unchanged behavior preserved.

`AGENT=1 bun test src/core/push.test.ts` → 22 pass, 0 fail.

Full suite has 3 pre-existing failures unrelated to this change (skill.test.ts requires a release binary not built locally; acid-references.test.ts has a Chinese-path encoding issue specific to my dev env).

Follow-up

A second PR (feat(push): support .acaiignore) builds on this one to add user-defined ignore patterns with gitignore-style syntax. Filed separately for clarity.

🤖 Co-authored-by Claude