ttl: add diagnostic severity model and non-deterministic task warnings

[kdy1]

Summary

Diagnostics currently represent only error-like records and do not carry severity levels or warning-class signals for non-deterministic task patterns.

Evidence

Contract Evidence:

• docs/project-ttl-language.md:230
• cmds/ttlc/internal/diagnostic/diagnostic.go:5

Current Gap

Diagnostics currently represent only error-like records and do not carry severity levels or warning-class signals for non-deterministic task patterns.

Proposed Scope

• Not documented yet.

Acceptance Criteria

• Extend diagnostic model with explicit severity (for example: error/warning/info).
• Introduce warning diagnostics for non-deterministic task patterns per documented contract.
• Preserve command status behavior so warnings do not incorrectly force fatal failure.
• Update tests to validate mixed diagnostic severities.

Test Scenarios

• Pure warning case returns warning diagnostics without runtime crash.
• Error + warning case preserves failure semantics with both records present.
• Existing error-only flows remain backward-compatible.

Out of Scope

• IDE/language-server integration.
• Heuristic warning families not tied to deterministic contract.