docs: refactor README for clarity and scannability

[marco-spagn]

• Flatter hierarchy with bold section headers for quick scanning
• Quickstart stays prominent at the top
• Features consolidated into readable prose grouped by area
• Architecture diagram and API surface tables simplified
• Development section organized by use case (essential, full stack, CI, feature smokes)
• Repository layout reduced to essentials (full details in docs/CODEBASE.md)
• Removed redundant target table (duplicated Makefile content)
• Added Cognitive Graph and MCP to feature list
• More whitespace, shorter paragraphs, linked docs for deep dives

Summary

Type of change

• Bug fix
• New feature / API change
• Documentation
• Refactor / tooling / CI
• Breaking change (describe migration)

Checklist

• make test (and make lint for Go changes)
• make ci-like-github or targeted job parity (make act-lint, make act-test, make act-integration-smoke) for CI-touching changes
• Integration tests updated if behavior/API changed (-tags=integration where relevant)
• CHANGELOG.md updated under [Unreleased] if user-visible
• Version bump in internal/version/version.go + docs/openapi.yaml + Helm appVersion if releasing API version (smoke picks up Tag automatically)
• OpenAPI / docs/grpc-vs-http.md / SDK HTTP-API.md updated for API changes

Test plan

[github-actions]

✅ Test coverage: 69.34%

Global statement coverage; threshold = 39% (scripts/ci_coverage_check.sh).

Package	Stmts	Covered	Coverage
config	64	56	✅ 87.5%
crypto	99	87	✅ 87.9%
database	16	15	✅ 93.8%
embedding	125	118	✅ 94.4%
event	273	206	✅ 75.5%
eventschema	23	21	✅ 91.3%
grpc	819	624	✅ 76.2%
handler	868	579	✅ 66.7%
metrics	27	18	✅ 66.7%
middleware	180	152	✅ 84.4%
model	95	66	✅ 69.5%
ratelimit	38	28	✅ 73.7%
repository	937	708	✅ 75.6%
service	335	260	✅ 77.6%
telemetry	26	24	✅ 92.3%
webhook	131	43	🟡 32.8%
worker	503	156	✅ 31.0%
Global	4559	3161	✅ 69.3%

_{Generated by scripts/ci_coverage_check.sh — bump floors in the same PR that lifts coverage.}