docs: add regulated-industries — structural fit of gitagent for compliance-heavy domains

[Priyanshu-Priyam]

Adds docs/regulated-industries.md — documenting the structural fit between gitagent's git-native architecture and regulated industries that have maker-checker controls, audit requirements, and exception workflows.

Proposed by @shreyas-lyzr in the GitClose review.

What's in the doc

• The isomorphism table (maker-checker = PR, audit trail = git log, SOD = branch protection, etc.)
• When the fit is strongest — domains with recurring workflows, repeating exceptions, and a clear maker/checker separation (finance, legal, healthcare, insurance, regulatory reporting)
• Reference implementation: GitClose — three gitagent-standard agents closing the January 2025 books of a fictional engineering firm, every agent action a commit, every approval a merged PR
• Extension guide — how to apply the pattern to a new domain

On the spec side

The compliance block (segregation_of_duties, supervision, recordkeeping) is the natural complement on the schema side. GitClose currently uses metadata for these fields — happy to follow up with an RFC if schema additions are wanted for finance/regulated agent types.