fix: resolve Issue #12 — ac.mjs no longer silently falls back to source repo

[tak2-08]

Summary

Fixes #12, part (A) only — the critical data-pollution bug.

tools/ac.mjs resolveRoot() previously fell back to new URL('../agent-context', import.meta.url) when the current working directory had neither agent-context.config.json nor agent-context/. When invoked from a wrong/empty directory (e.g. by an automation script that didn't set cwd, or via npx), it silently wrote user entries into the package source (or npx cache) and regenerated index.json/graph.json — polluting the install.

Now resolveRoot() prints a clear bilingual error and process.exit(1) instead of falling back:

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>

Dogfooding from the repo root still works because the repo root itself contains agent-context.config.json + agent-context/ (cwd detection branch unchanged).

Verification

• Repro (pre-fix): git show HEAD~1:tools/ac.mjs showed the silent new URL('../agent-context', ...).pathname fallback.
• Post-fix: running ac.mjs from a clean dir with no project → exit code 1 + bilingual error; git status of the repo shows no pollution of agent-context/ (index.json/graph.json untouched, no new files).
• Regression: cwd-with-project (repo root) still creates entries correctly.
• node --check tools/ac.mjs passes.
• node tools/e2e-workflow.mjs → all 6 steps pass.

Out of scope (filed as future work)

Part (B) of #12 (synonym dict expansion + search-lite padding/threshold) is not addressed here, per task scope.
The same silent directory fallback also exists in ac-watch.mjs and ac-fts.mjs (and many tools read the source agent-context.config.json when cwd has no project) — noted for a follow-up issue, not changed in this PR.

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