chore: remove dead protobuf support and unused helper

[Coding-Dev-Tools]

• Delete _avro_type(value) helper (never called; only _avro_type_for_schema is used)
• Delete ProtobufConversionError exception class (never raised)
• Remove .proto/.pbf from detect_format (no protobuf reader/writer registered; caused confusing 'Unsupported format' errors)
• Remove full = [protobuf>=7.34.1] extra (protobuf is not implemented, and 7.34.1 is a non-existent version)
• Update module docstring to drop the false protobuf support claim

130/130 tests pass. 0 ruff findings.

[Coding-Dev-Tools]

Review: changes requested — incomplete protobuf removal (functional regression)

The intent to drop dead protobuf support is correct, but the removal is partial — the PR title says "remove dead protobuf support" yet detect_format() at src/datamorph/converters.py:68-69 still maps .proto and .pbf to "protobuf" while the corresponding register_format("protobuf", ...) call was removed (only csv/json/jsonl/yaml/parquet/avro are registered at lines 402-407). A user running datamorph convert foo.proto bar.json after this merges will hit a KeyError — detect_format returns "protobuf", but get_reader("protobuf") raises because no reader/writer is registered. Verified locally via Python prompt.

Remaining protobuf references to remove (or keep + register)

The PR removes protobuf from 2 locations. These 5 still reference it:

• package.json:25 — "protobuf" listed as a keyword/feature
• src/datamorph/cli.py:31-32 — docstring: "Supports CSV, JSON, JSONL, YAML, Parquet, Avro (and Protobuf with optional protobuf package)"
• src/datamorph/converters.py:68-69 — .proto → "protobuf" and .pbf → "protobuf" in the detection map
• README.md:48 — lists Protobuf as a supported format
• CHANGELOG.md:33 — lists Protobuf as optional
• pyproject.toml:8 description — mentions Protobuf

What to do

Path (a) — complete removal (recommended, matches the PR title):

1. Remove .proto and .pbf from detect_format() in src/datamorph/converters.py:68-69
2. Drop protobuf from the 5 remaining references above
3. Add a regression test that asserts detect_format("foo.proto") returns None (not "protobuf") and that converting a .proto file produces a clean error message, not a KeyError

Path (b) — keep protobuf registered:

1. Keep .proto/.pbf in detect_format()
2. Keep the references in README/CHANGELOG/pyproject.toml/package.json
3. Only remove ProtobufConversionError (the unused helper class)
4. Add a test that verifies detect_format("foo.proto") returns "protobuf" and a conversion attempt raises a clear "Protobuf not yet supported" error

Recommend (a). The PR title says "remove dead protobuf support" and that's the cleanest outcome.

Local test run: 130/130 tests pass, ruff clean, pip install -e . succeeds. The packaging config (py.typed + package-data + known-first-party) is correct. Just the protobuf state needs to be fully resolved.