Support tags and !! shorthand

[MariusStorhaug]

Why

YAML 1.2.2 lets nodes be explicitly typed via tags, but any !/!! prefix is currently treated as plain text, so explicitly-tagged documents parse incorrectly.

What

Parse and honour the standard !! shorthand tags: !!str, !!int, !!float, !!bool, !!null to force scalar resolution; !!seq, !!map to force collection kind; !!set exposed as a hashset-like structure; !!omap exposed as an [ordered] hashtable; and verbatim (!<tag:...>) and local (!foo) tags accepted.

Spec examples covered

• Example 2.21 — Miscellaneous (typed forms with !!str)
• Example 2.23 — Various Explicit Tags
• Example 2.24 — Global Tags
• Example 2.25 — !!set
• Example 2.26 — !!omap

[MariusStorhaug]

What changed: Reformatted the issue body to the MSXOrg Issue Format — a local Why plus an outcome-level What/acceptance criteria. Removed the Technical Decisions and Implementation Plan sections (and other implementation-tradeoff notes).
Why: Issue bodies should state local purpose and observable outcome without leaking implementation detail into the outcome-level description; durable technical decisions belong in the PR/design, not the tracking issue.
Follow-up: None.