Consolidate the 31 root-level implementation summary markdown files into docs/

[RUKAYAT-CODER]

Overview

The repository root contains 31 tracked markdown files, most of them per-feature implementation reports: ASSIGNMENT_COMPLETION_REPORT.md, ASYNC_WORKERS_IMPLEMENTATION.md, COMPLETION_SUMMARY.md, CURRENCY_CONVERSION_IMPLEMENTATION.md, four separate ENV_* documents, five INCIDENT_MANAGEMENT_* documents, three ROUTING_* documents, two GRACEFUL_SHUTDOWN_* documents, and more. These are point-in-time PR narratives rather than reference documentation; they duplicate and contradict each other (four env documents, three routing status reports), and they bury README.md and CONTRIBUTING.md — the two files a new contributor actually needs — in noise. A docs/ directory already exists and is where this material belongs.

Specifications

Features:

• The repository root holds only README, CONTRIBUTING, CHANGELOG, and licence.
• Reference documentation is organised by topic under docs/ with no contradictory duplicates.

Tasks:

• Move each implementation report under docs/implementation-notes/ preserving git history.
• Merge the four ENV_* documents into one environment reference and the three ROUTING_* documents into one routing document, deleting the superseded copies.
• Do the same for the five INCIDENT_MANAGEMENT_* and two GRACEFUL_SHUTDOWN_* files.
• Add a docs/README.md index and link it from the root README.md.
• Delete reports describing work that is fully documented in the code or the changelog.

Impacted Files:

• README.md
• docs/
• CONTRIBUTING.md

Acceptance Criteria

• At most four markdown files remain in the repository root.
• No two documents in docs/ describe the same subject with contradictory content.
• docs/README.md indexes the reorganised material.
• All internal documentation links resolve.