feat: add line-count budget check for global rule modules

After composing AGENTS.md, counts lines in each global rule module and
warns to stderr when total (default 350) or per-module (default 30)
budget is exceeded. Budget limits are configurable via agent-ruleset.json
`budget` field. --quiet suppresses the warning; --json includes budget
data in the output object.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: metyatech