Harden Cortex for open-source launch

[VamikaSinghal]

What changed

• rewrites the README and contributor path around the shipping local-first product
• adds verified screenshots, demo assets, issue templates, code map, and connector guide
• hardens health/readiness behavior, standalone networking, vector-index migration, and context pin atomicity
• adds hash-locked Python dependency graphs and release-runtime CI coverage
• moves the archived prototype into legacy/ and strengthens deployment defaults

Why

This removes installation, documentation, release, and security friction found during independent architecture, contributor, and operations reviews before a broader open-source launch.

Verification

• full backend: 2,212 passed, 6 skipped, 444 subtests
• make check
• Python SDK: 29 passed
• TypeScript SDK: 26 passed
• retrieval evaluation: 163 cases, top-1 1.0, recall@3 1.0
• Python/npm dependency audits: zero known vulnerabilities
• configured Bandit gate: no medium/high findings
• workflow YAML, shell syntax, diff hygiene, and credential scan passed
• bundled runtime and signing checks were exercised locally during the review