ci(e2e): skip shards for backend, sync, and scripts-only pull requests

[tyler-dane]

Fixes: no issue. Second fix from the CI audit started in #3407.

What and why

The Playwright suite boots the web dev server (packages/web, which imports only packages/core) against stubbed /api/** routes. Neither e2e/, playwright.config.ts, nor packages/web/dev.ts imports from packages/backend, packages/sync, or packages/scripts, so a PR that touches only those packages cannot change an e2e result. In the audit window, 50 of 232 PR e2e runs (22%, across 33 PRs) were such PRs: 200 shard jobs and about 4 minutes of PR wall clock each, for no signal.

detect-code-changes.sh gains an e2e output next to code: false when every changed file is under those three packages or is docs. The e2e-shard job gates on it; the e2e gate already reports Success when the shards are skipped, so the required check still reports and docs-only handling is unchanged (#2938, #2940). merge_group and push events still report true, so every merge is tested end to end before and after it lands on main. The Unit workflow still reads only code.

Tests cover: backend+sync+scripts+docs skips e2e only; backend plus any one of core, web, e2e/, playwright.config.ts, bun.lock, or the e2e workflow runs everything; unverifiable file lists run everything; the two workflows gate on the intended outputs.

Verify

Selected packages: scripts
Checks run: type-check, lint, knip
Failed: test:scripts:fast
VERDICT: FAIL

The three failures are the same as in #3407: agent-loop-next.sh uses mapfile, which macOS /bin/bash 3.2 lacks. Not touched by this diff; passes on the Linux runner. All 9 detect-code-changes tests, the other scripts tests, type-check, lint, and knip pass locally. CI on Linux is the arbiter.

🤖 Generated with Claude Code