docs: align README with coding-agent reliability positioning

[paceaitian]

What changed

• restore the agreed public positioning: deterministic workflow enforcement plus persistent engineering memory
• describe PACEflow as a reliability model for long-running coding agents before identifying the current Claude Code implementation
• add a concise Why PACEflow? section focused on workflow continuity failures
• make the trust boundary explicit with Deterministic gates, not deterministic intelligence

Why

The first international README over-compressed the agreed framing and opened by defining PACEflow as a Claude Code plugin. That was technically accurate but recreated the implementation-first positioning the international landing page was intended to avoid.

This correction keeps the concise README structure while aligning its opening with the project's broader, evidence-backed purpose. The repository About description and Topics have also been updated separately to use the same positioning.

Impact

Documentation and repository metadata only. No plugin runtime, hook behavior, schema, command, or migration code changes.

Validation

• node tests/run-all.js — 8/8 suites passed; hooks-e2e 482/482
• Claude plugin manifest validation passed
• all relative Markdown link targets exist
• required positioning phrases verified mechanically
• git diff --check passed