chore: add tessl registry for AI agent library docs

[KRRT7]

Summary

• Add AGENTS.md to .gitignore (auto-generated by tessl install)
• Add explanatory comment in CLAUDE.md for the tessl-managed section

What is Tessl?

Tessl is a registry that provides curated, AI-agent-friendly documentation for libraries. When working with AI coding assistants (Claude Code, Cursor, etc.), the agent can query the Tessl MCP server to get up-to-date API docs and usage patterns for dependencies instead of hallucinating or using outdated knowledge.

We're using it to provide accurate library documentation for our dependencies (pytest, click, libcst, pydantic, etc.) so AI agents can write better code when working on this codebase.

Example context provided

When an agent queries for libcst usage, it gets structured docs like:

# Core parsing functions
def parse_module(source: Union[str, bytes], config: Optional[PartialParserConfig] = None) -> Module: ...
def parse_statement(source: str, config: Optional[PartialParserConfig] = None) -> Union[SimpleStatementLine, BaseCompoundStatement]: ...

# Visitor framework
class CSTVisitor:
    def visit(self, node: CSTNode) -> None: ...
    def leave(self, node: CSTNode) -> None: ...

class CSTTransformer:
    def visit(self, node: CSTNode) -> None: ...
    def leave(self, original_node: CSTNode, updated_node: CSTNode) -> CSTNode: ...

# Pattern matching
def matches(node: CSTNode, matcher: Union[CSTNode, MatcherPattern]) -> bool: ...
def findall(tree: CSTNode, matcher: Union[CSTNode, MatcherPattern]) -> Sequence[CSTNode]: ...

This helps agents write correct code without needing to search through source files or guess at APIs.