feat(brief): add customizable context aware brief

[AbysmalBiscuit]

Add ability for brief to add specialized messages based on context.

Eg, if an env var is present or has certain value.

Or if agent does something in certain folder.

Eg if codex does something in a sub folder with an AGENTS.md, it loads and injects the AGENTS.md file into context.

It should support a standardized rules interface:

pub enum Category {
    CrashOrHang,
    LogicError,
    Performance,
    Security,
    ErrorHandling,
    Readability,
    CodeStyle,
    Maintainability,
    Testability,
    BestPractice,
}

pub enum Task {
    CodeReview,
    CodeGeneration,
    CodeQuestions,
}

pub enum Language {
    Bash,
    CPP,
    CSS,
    CSharp,
    Dart,
    Elixir,
    Go,
    HTML,
    Haskell,
    Java,
    JavaScript,
    Kotlin,
    Lua,
    Perl,
    PhP,
    Python,
    R,
    Ruby,
    Rust,
    SQL,
    Scala,
    Shell,
    Swift,
    TypeScript,
}

pub enum Severity {
    Must,
    Should,
    Can,
}

#[derive(Debug)]
pub struct Rule {
    pub categories: Vec<Category>,
    pub description: String,
    pub directory: String,
    pub id: String,
    pub languages: Vec<Language>,
    pub scope: Scope,
    pub severity: Severity,
    pub source_file: String,
    pub tasks: Vec<Task>,
    pub title: String,
}