fix(build): suppress Turbopack over-bundling warning from agentSkills generator (#6582)

[diegosouzapw]

Closes #6582

Root cause

src/lib/agentSkills/generator.ts:247 builds outputBase as path.isAbsolute(outputDir) ? outputDir : path.join(process.cwd(), outputDir), where outputDir is a runtime function parameter, not a compile-time literal. Turbopack's build-time file-tracing analyzer can't statically narrow the dynamic readdirSync/rmSync/readFileSync/writeFileSync call sites a few lines below, so it falls back to a project-wide glob — once per Next.js entry point that imports the module (/api/agent-skills/generate, /api/cli-tools/pi-settings). #6366's commit message claimed to "anchor the base path with a literal" but the shipped code never did; a later "#6366 regression" fix only added an isAbsolute() guard for absolute test paths.

Fix

Since the fs access is legitimate and bounded (skills/<id>/SKILL.md, ~48 known IDs), rather than fight the analyzer, next.config.mjs's turbopack.ignoreIssue (Next.js 16.2+) now suppresses this specific, known-benign diagnostic — mirroring the existing webpack.ignoreWarnings/isNextIntlExtractorDynamicImportWarning precedent already in the same file for the webpack path.

TDD evidence

No unit-testable repro exists for a Turbopack compiler diagnostic (confirmed by prior /triage-fix-bugs analysis), so the regression guard is a config-shape assertion in tests/unit/next-config.test.ts:

• RED (base next.config.mjs, no ignoreIssue rule):

  ✖ turbopack.ignoreIssue suppresses the agentSkills over-bundling warning (#6582)
    AssertionError: expected turbopack.ignoreIssue to be an array
  

• GREEN (with the fix):

  ✔ turbopack.ignoreIssue suppresses the agentSkills over-bundling warning (#6582)
  

Gates run (green)

• node --import tsx/esm --test tests/unit/next-config.test.ts — 7/7 pass
• Sibling next.config.mjs consumers (dev-origins-config, redirects-cli-renames, mitm-stub-alias-6344, csp-lan-ws-5083, api/root-catchall-json-404) — 21/21 pass
• node scripts/check/check-file-size.mjs — OK, no violations on touched files
• node scripts/check/check-complexity.mjs — OK (2052 violations, baseline 2053)
• node scripts/check/check-cognitive-complexity.mjs — OK (884 violations, baseline 884)
• npx eslint --suppressions-location config/quality/eslint-suppressions.json next.config.mjs tests/unit/next-config.test.ts — exit 0
• node scripts/check/check-changelog-integrity.mjs — OK
• npm run typecheck:core — pre-existing base-red on open-sse/services/compression/engines/omniglyphAdapter.ts/stats.ts (missing omniglyph module, from feat(compression): omniglyph engine (context-as-image, Fable 5 direct) — stack + single mode #6556), unrelated to this diff (confirmed: git diff origin/release/v3.8.47 --stat touches only CHANGELOG.md, next.config.mjs, tests/unit/next-config.test.ts)

No release-freeze active at time of PR (gh issue list --label release-freeze --state open empty).