Add TDL-0004: error vs warning semantics and phase boundaries

[ronpal]

Summary

• Adds TDL-0004 documenting the toolkit's error/warning model: commands run to completion and aggregate issues at the next phase boundary, with ToolkitError reserved for unrecoverable conditions that abort immediately.
• Acknowledges the three parallel severity systems we have today (legacy tk_warnings.SeverityLevel, build v2 numeric InsightDefinition, dataio/logger.Severity) and the cognite.neat._toolkit_adapter mapping where the Error/Warning naming bug originates.
• Defines what a phase boundary is per command and why mutation phases stay fail-fast.

Draft for review — the open naming question (WARNING/NOTICE/HINT vs alternatives) is still being settled with @magnus-schjolberg.

Related

• CDF-27907 — terminology / visual language for build output
• CDF-27968 — reclassify HIGH-severity warnings as collected errors (uses this TDL as the contract)

Test plan

• Review wording with @magnus-schjolberg and @anders-albert
• Confirm naming choice (WARNING/NOTICE/HINT) before implementation work begins on CDF-27968

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