chore(lint): reduce backend detekt baseline to 29

[Pengzhike]

Summary

• reduce backend detekt baseline from 38 to 29 by refactoring backend lint hotspots and removing one unused PDF font probe
• tighten exception handling in backend AI/todo flows and ACP test coverage without changing the existing runtime contracts
• archive slices 149-155 and update the active lint baseline reduction tracker

Validation

• git diff --check
• ./gradlew silkLint
• ./gradlew :backend:test

Docs

• updated docs/context/planning/exec-plans/active/lint-baseline-reduction.md
• added completed slice records 149-155 under docs/context/planning/exec-plans/completed/
• checked ARCHITECTURE.md, docs/context/quality/*, and README.md; no further doc updates were needed