fix(kotlin): parse ktlint --reporter=json output correctly

[violinyanev]

Problem

The kotlin plugin declares its ktlint tool with fmt=\"json\", which maps to
the generic parse_json parser. That parser expects a flat list of
objects with top-level file/line/message keys (the shape most linters
use), but ktlint --reporter=json nests violations per file:

[
  {
    "file": "src/main/Foo.kt",
    "errors": [
      {"line": 10, "column": 5, "message": "Newline expected", "rule": "standard:class-signature"}
    ]
  }
]

Since parse_json looks for a top-level "line"/"message" key that
doesn't exist on these file objects, every entry is silently dropped. The
result: ktlint_violation always returns zero entries even when ktlint
exits non-zero with real violations, and the scan reports a permanent

Coverage reduced (ktlint_violation): ktlint tooling unavailable (tool_failed_unparsed_output)

warning — so Kotlin projects never actually get ktlint findings out of
desloppify, regardless of whether ktlint is installed and working.

Repro (any Kotlin project with ktlint on PATH and at least one lint
violation):

$ desloppify scan --path .
...
Coverage reduced (ktlint_violation): ktlint tooling unavailable (tool_failed_unparsed_output)

Fix

Add a dedicated parse_ktlint parser that understands ktlint's nested
file/errors shape (one entry per error, with the rule id appended to the
message), register it in PARSERS, and switch the kotlin plugin's tool spec
to fmt=\"ktlint\" instead of the generic \"json\".

Verified against a real multi-module Kotlin (Android) project:

• Before: ktlint_violation coverage reduced, mechanical "Code quality"
  score understated (e.g. 60.7%).
• After: no coverage warning, real ktlint findings show up in desloppify next/show, and "Code quality" score reflects them (e.g. 90.9%).

Added unit tests (TestParseKtlint) covering rule-suffix formatting,
multiple files/errors, files with no errors, empty output, and invalid
JSON. Full test suite passes locally (5200 passed, 1 skipped).