feat(marshal-receiver): add marshal-receiver rule

[roidelapluie]

Add a new rule that enforces correct receiver types for Marshal/Unmarshal methods: MarshalJSON/MarshalYAML should use value receivers, while UnmarshalJSON/UnmarshalYAML should use pointer receivers.

see #1706

[Copilot]

Pull request overview

Adds a new marshal-receiver lint rule to detect incorrect receiver types for common JSON/YAML marshal/unmarshal methods, addressing subtle serialization bugs described in #1706.

Changes:

• Implement MarshalReceiverRule to enforce value receivers for MarshalJSON/MarshalYAML and pointer receivers for UnmarshalJSON/UnmarshalYAML.
• Register the rule in the global rule set and document it in RULES_DESCRIPTIONS.md.
• Add fixture-based coverage and a dedicated rule test.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
rule/marshal_receiver.go	Implements the new receiver-type enforcement rule for marshal/unmarshal methods.
config/config.go	Registers the new rule in the list of available rules.
config/config_test.go	Updates the expected total rule count to include the new rule.
test/marshal_receiver_test.go	Adds a unit test running the rule against a new fixture.
testdata/marshal_receiver.go	Adds good/bad examples with expected failure messages for the new rule.
RULES_DESCRIPTIONS.md	Documents the new marshal-receiver rule and adds it to the rule index.