fix: resolve #14 (source-repo fallback) + #15 (synonyms & relevance gate)

[tak2-08]

Summary

Closes #14. Closes #15.

#14 — silent source-repo fallback removed (data-pollution fix)

Three tools previously resolved a project root by falling back to the package's own source agent-context/ when cwd was not an initialized project — polluting the package install / npx cache with user data.

• tools/ac-watch.mjs — resolveProjectRoot()
• tools/ac-fts.mjs — resolveProjectRoot()
• tools/agent-search-lite.mjs — resolveProjectRoot()

All three now mirror the ac.mjs (#13) pattern: when cwd has neither agent-context.config.json nor agent-context/, they print a bilingual error and process.exit(1) instead of touching the source repo:

agent-context가 초기화되지 않았습니다. 먼저 'agent-context-init.mjs --yes' 를 실행하세요.
(agent-context not initialized in cwd; run 'agent-context-init.mjs --yes' first.)
cwd: <cwd>

No new URL('../agent-context'…) fallback remains in any tool.

#15a — business/domain synonyms

agent-context.config.json search.synonyms gained a billing↔payments↔charge↔invoice↔결제 cluster (bidirectional), merged with (not replacing) the existing jwt/token, auth/authentication/인증, bug/issue/버그, race/경쟁조건, pagination/페이지네이션/cursor pairs.

#15b — relevance gate (suppress weak/no-match padding)

A new search.minHitWeight config key plus a --min-weight N CLI flag gates results by hit weight. Queries whose best match is below the threshold now return a bilingual guidance message ("관련 결과 없음 — 상위 레벨로 확장 검색 필요 / no related results; expand to a higher level") instead of padding the answer with unrelated entries.

• minHitWeight = 1 (legacy): historical padding behavior.
• minHitWeight >= 2 (this PR default): weak/no-match padders are suppressed.

Verification

• node --check passes on ac-watch.mjs, ac-fts.mjs, agent-search-lite.mjs.
• node tools/e2e-workflow.mjs → 6/6 steps passed (at both minHitWeight 1 and 2).
• node tools/agent-search-lite.mjs --benchmark → identical 60% hit rate at both values; the two misses ("overall flow", "pagination") are genuine zero-match queries, not caused by the gate.
• Pollution repro from an uninitialized dir: all three tools exit 1 with the bilingual error and create no agent-context/ data in the source tree.
• git status shows only the 4 intended files changed.

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