Improve turbo:check granularity and content validation

[gtbuchanan]

Summary

turbo:check currently only validates existence of turbo tasks and
package scripts. It doesn't check content or distinguish between task
and script ignores.

Current limitations

• Existence-only: only flags missing tasks/scripts, not modified
  content. Can't distinguish intentional overrides from accidental
  deletions.
• No granularity: --ignore compile:ts ignores both the turbo
  task and the package script, even if only the script is intentionally
  different.
• CLI-only: ignores must be passed as args every time, not
  persisted in config.

Possible improvements

• Validate task/script content against expected baseline
• Separate --ignore-task and --ignore-script flags
• Support a .turbocheck or turbo:check field in package.json for
  persisted ignores
• Report modified values as info (not drift) so consumers see what
  they've customized

Context

Added during #17 (Turborepo integration). The current implementation
is sufficient for detecting missing config but not for auditing
intentional deviations.