fix: run ESLint directly on Next.js 16+ in next_lint tool

[szupzj18]

Problem

Fixes #709

Next.js 16 removed the next lint subcommand. The hardcoded npx --no-install next lint --format json in NEXTJS_SPEC is now parsed as next <dir> and exits without linting:

Invalid project directory provided, no such directory: /path/to/project/lint

The tool phase records reduced coverage (parser_error) and reports zero lint issues, so a Next 16 codebase reads as lint-clean.

Fix

• next_lint_cmd(scan_root) resolves the command from the installed Next.js major (node_modules/next/package.json, not the declared range — ^15.0.0 can resolve to 16). On >= 16 it runs npx --no-install eslint --format json . directly; ESLint's JSON output has the same shape the next_lint parser already consumes.
• ToolIntegration.cmd now accepts str | Callable[[Path], str]; _framework_tool_phase resolves it per scan root.
• Falls back to next lint when Next is not installed, the version is unreadable, or the major is < 16.

Tests

• next_lint_cmd: Next 16 → ESLint command; Next 15 → next lint; missing install → fallback; malformed package.json → fallback.
• Tool-phase integration: a project with installed next@16 executes the ESLint command (verified RED with the old static command — argv contained lint); next@15 still executes next lint.
• desloppify/tests/detectors + tests/scan: 974 passed. Full suite: only the pre-existing test_bash_unused_source_directive_is_flagged failure (fails on main without this change too).

Checklist

• Tests added (RED before fix, GREEN after)
• Existing tests pass
• Syntax lint clean (ruff --select E9,F63,F7,F82 scope)

[szupzj18]

Closing for now — the repo appears unmaintained (no merges since 2026-03, no response on these PRs). The fixes remain valid against the linked issues; happy to reopen if maintainer activity resumes.