feat: implement rule-based field validation

[ADernild]

feat: implement rule-based field validation

Adds a conditional rule system that enforces constraints on commit fields
after all values are collected.

What's new

Rule engine — Rules consist of a condition and one or more actions.
Conditions support eq, neq, in, exists, and logical combinators (and, or,
not). Actions support require, forbid, and equals, each with an optional
custom message. Violations are collected and reported together with
human-readable messages that include the triggering condition.

Rule validation — Config validation now checks that all fields
referenced in rule conditions and actions exist in the schema, catching
mistakes at load time.

Config refactor — Render logic extracted into render.rs, rules into
rules.rs, and shared tests consolidated into tests.rs. schema.rs is now
focused on data types and structural validation only.

Example

[[rule]]
name = "feat-requires-scope"

[rule.condition]
eq = ["type", "feat"]

[[rule.action]]
require = { fields = ["scope"], message = "Feature commits must have a scope" }