fix(container): multi-turn FIFO persistence

[mariuspruvot]

Summary

• Root cause: --input-format stream-json exits after each conversation turn by design (exit 0, terminal_reason: "completed"). The single long-lived process approach never worked for multi-turn.
• Fix: Replace with per-turn invocations -- first turn uses claude -p "$PROMPT", subsequent turns read from FIFO and use claude -c -p "$CONTENT" (--continue) to resume with full context from Claude's local session store.
• Also: Simplify send_message() to write raw content instead of stream-json wrapper; ensure newline-terminated FIFO writes for read compatibility; add write_to_container to FakeDockerClient and 3 new tests for send_message.

Changes

File	What
infra/docker/claude-runner/entrypoint.sh	Per-turn loop with claude -c -p, signal handling for clean shutdown
apps/api/src/helprs/modules/container/service.py	Raw content in send_message(), newline-terminated write_to_container()
apps/api/tests/modules/container/test_service.py	write_to_container on FakeDockerClient, 3 new TestSendMessage tests
CLAUDE.md	Document multi-turn architecture, updated stream-json protocol notes

Test plan

• All 31 container service tests pass (including 3 new send_message tests)
• Full test suite: 226/228 pass, 2 pre-existing failures unrelated to this change
• Ruff + ESLint clean
• Manual e2e: start a challenge-me session, answer Q1, verify Claude continues with Q2 instead of exiting