docs(spec): add high-level conventions — paths, NULL defer

[CGMossa]

Unique design choice, where R API is treating NULL as default value by library. This means, that NULL is not a fancy way of writing 0, because it may result in non-zero settings.

AI-written details

Summary

• Adds two short subsections under the "High-level overview" recording rules that already exist in the implementation but were not written down:
  • Relative paths vs. absolute paths — every dvs command accepts both forms; project-scope / symlink resolution is the library's responsibility, not the caller's.
  • R package — pass NULL in dvs_* functions when you want to defer to the library's default value.
• Carved out of docs(spec): remaining audit edits (errors, init/add/get/status text + R signatures, audit-log JSON, threading) #149 (spec audit april).
• The original spec-audit pass also included a "Rust library must never use std::io::{Stdin, Stdout, Stderr}" subsection. Omitted here on purpose — the broader no-CLI/no-R-package coupling invariant on the same topic already landed on main via spec: require dvs core to be free of CLI/R-package concerns #195 / 9df3750, and re-stating a narrower form alongside it would just create drift.

Test plan

• specs.md renders cleanly on GitHub
• No dvs_* R wrapper in dvs-rpkg/R/ contradicts the NULL-defer rule
• No conflict with the library-isolation paragraph already on main

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Drafted by Claude (claude-opus-4-7). Reviewed by the author.