fix(learnings): preserve current entries in cross-project search

[jbetala7]

Fixes #1618

Summary

• tag learnings JSONL rows by source file before parsing them
• apply the cross-project trust gate only to rows loaded from other projects
• add regressions for current-project observed entries and trusted/untrusted foreign entries

Root cause

gstack-learnings-search --cross-project inferred cross-project rows with !line.includes(slug). Current-project learnings do not normally include the project slug in their JSON body, so observed/inferred current learnings with trusted:false were treated as foreign and dropped.

Verification

• bun test test/gstack-learnings-search.test.ts
• bun test test/learnings.test.ts test/gstack-learnings-search.test.ts
• direct clean-home repro: --cross-project --query current now prints the current observed learning, while --query foreign only prints the trusted user-stated foreign learning
• git diff --check origin/main...HEAD
• git merge-tree --write-tree HEAD origin/main

Collision check

Searched open issues/PRs for cross-project learnings, gstack-learnings-search cross-project trusted, current project learnings --cross-project, cross-project trust gate learnings, gstack-learnings-search --cross-project, bin/gstack-learnings-search, and issue #1618.

Open PR #940 touches bin/gstack-learnings-search, but it retains the same line.includes(slug) cross-project classification and is about legacy schema normalization, so it does not cover this bug.