Architecture R31: eliminate null absence outside Java persistence

Follow-up to the 2026-10-18 architecture audit, linked to PAR-449 and the completed architecture rebuild PAR-408. Rules are in docs/architecture.md. Audit baseline: commit 08d99ee; all 18 architecture checks passed, which does not establish full compliance.

Baseline scan found 19 explicit return null; statements in 16 Java files outside persistence; this is a lower bound, since it excludes .orElse(null), nullable fields and arguments. Examples: issues/usecase/UpdateIssue.statusName, triage/usecase/ApplyDecision.firstNonBlank, MCP parsers, webhook helpers, and platform/mcp/McpRequestContextExtractor.

Acceptance:

• Inventory null construction/propagation outside persistence, separating application-owned absence from framework-required nullable contracts.
• Represent application absence with Result, Optional or empty collections as appropriate; confine nullable database values to persistence.
• Document narrowly scoped framework interop boundaries if unavoidable, rather than treating every nullable callback as an identical fix.
• Preserve missing/invalid input error semantics for HTTP, MCP and Discord.
• Add a maintainable regression check or nullness enforcement that catches violations beyond literal null returns; do not add suppression baselines.
• Run affected tests and architecture checks. Coordinate with the issue-write persistence boundary follow-up.