docs(serial-gate): sync system showcase + clean stray tags (ORCH-124)

Addresses reviewer REQUEST_CHANGES (run 768) on ORCH-124 — docs-only,
no src/tests touched, fix scope unchanged.

P1: update docs/overview/ showcase for the new serial-gate "pause without
blocking" axis (changed task-routing functionality, ORCH-011/ORCH-079):
- tech-pipeline.md: FIFO exception "pause without blocking" next to freeze
- tech-data-model.md: durable signal tasks.paused_at on the Task row
- tech-observability.md: paused/reason in serial_gate GET /queue block +
  operator endpoints POST /serial-gate/pause|resume

P2: strip leaked tool-call trailing tags (</content>/</invoke>) from 4
golden-source docs of this PR (06-adr/ADR-001, adr-0051,
08-data-requirements.md, 10-tech-risks.md).

CHANGELOG "Доки" bullet extended accordingly. Full suite green (2178 passed);
test_system_docs.py green (machine-checked showcase facts intact).

Refs: ORCH-124
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/architecture/adr/adr-0051-serial-gate-pause-without-blocking.md

@@ -108,4 +108,3 @@ rebase-машинерии нет: отложенный срез ветки (ORCH
 - Данные: `docs/work-items/ORCH-124/08-data-requirements.md`
 - Связанные: adr-0017 (serial-gate ORCH-088), adr-0026 (терминал `{done,cancelled}` ORCH-090),
   adr-0015 (task-deps), adr-0027 (merge-актор rebase/retry ORCH-093), adr-0042 (merge-gate re-test ORCH-110)
-</content>