docs: rename project to Plexus and rewrite README

[Sardor-M]

Summary

Rename the project from CoreMQ to Plexus and rewrite the README around the new identity.

Plexus — anatomical term for a network of nerves (e.g. solar plexus, brachial plexus). Direct metaphor for what an MQTT broker does: act as the central junction that every signal routes through. Distinctive in the MQTT/messaging space, no major collisions, technical without being pretentious.

This PR is README-only. The Cargo crate (coremq-server), the npm package (coremq-dashboard), the config paths (/etc/coremq/), and the GitHub repo name are unchanged. A follow-up PR will handle the rename across code/config — kept separate so the rebrand decision can be reviewed independently of the mechanical renames.

Why a separate name

CoreMQ is technically accurate but generic — core + MQ reads as a placeholder. Five other projects use core in their messaging product name. Plexus is:

• Descriptive of function. A plexus is literally a routing network. The broker's job is routing; the name tells you what it does.
• Single-word. No MQ suffix needed — the architecture-as-name does the work.
• Distinctive. Not used in the MQTT/messaging space. Searchable.
• Technical, not marketing. Reads like infrastructure, not a startup landing page.

What's in the rewrite

Diff: +146 / −211 (357 → 305 lines, ~28% shorter, denser).

• New tagline. Opening paragraph explains the architectural choice behind the name (one Tokio engine, async channels, single-threaded event loop).
• Lead with At a glance — seven bullets covering the whole feature surface in 10 seconds.
• Merge the redundant MQTT Protocol Support + Multi-Transport Support sections — they were saying the same thing twice.
• Architecture: same diagram, but the prose now explains why the single-threaded engine matters (no long-held locks, no contention, easy to reason about) instead of just describing it.
• Performance and Security are paragraphs, not bullet lists. Drop marketing-speak ("zero-cost abstractions", "designed for IoT-scale workloads") that doesn't carry information.
• Topic Monitoring & REST Publish: kept the why-it-matters list and the curl examples; dropped the long JSON sample (callers can read the spec).
• QoS claims rewritten to match reality. Was: "QoS 0 (expandable to QoS 1/2)". Now: "QoS 0 in production, QoS 1/2 wiring in progress." This matches what's actually shipped — the recent packet_id panic fix (docs: add AGENTS, CONTRIBUTING, CHANGELOG, and skill playbooks #5 → ef67625) is part of the QoS 1/2 path, not full support.
• Roadmap: drop "WebSocket metrics endpoint" — already shipped and documented above (was a copy-paste from a stale roadmap).
• New Contributing section linking CONTRIBUTING.md, AGENTS.md, CHANGELOG.md. Note: those three files are added in docs: add AGENTS, CONTRIBUTING, CHANGELOG, and skill playbooks #5 — links resolve once both this PR and docs: add AGENTS, CONTRIBUTING, CHANGELOG, and skill playbooks #5 are on main.
• Honest callout that "Plexus" branding is ahead of the package rename, so readers don't get confused by coremq-server in the build commands.

Follow-up renames (separate PRs)

If the name lands well, the deferred mechanical changes are:

1. server/coremq-server/Cargo.toml → name = "plexus-server" (or "plexus"), update workspace members, fix cargo run -p invocations across docs.
2. client/package.json → "name": "plexus-dashboard".
3. Config paths: /etc/coremq/ → /etc/plexus/, C:\ProgramData\CoreMQ\ → C:\ProgramData\Plexus\. Migration shim or doc on how to move existing config.
4. Repo rename: mqttRust/coremq_rust → mqttRust/plexus. GitHub redirects from the old name.
5. Docker image name + docker-compose service name.

Each of these is independently reversible. None block this PR.

Test plan

• Read the new README top to bottom, look for stale CoreMQ references or broken links.
• Verify the architecture diagram still renders properly in GitHub's markdown preview.
• Confirm make build / docker compose up -d flows still work — README doesn't change them.
• If you'd rather the name be different, comment with the alternative; this PR is cheap to retitle.

Notes

• Pairs with chore(ci): refine Claude Code Action workflows #4 (workflow refinements) and docs: add AGENTS, CONTRIBUTING, CHANGELOG, and skill playbooks #5 (agent docs + skills + lockstep version). No file overlap with either, no merge conflicts expected. Suggested merge order: chore(ci): refine Claude Code Action workflows #4 → docs: add AGENTS, CONTRIBUTING, CHANGELOG, and skill playbooks #5 → Add Claude Code GitHub Workflow #3 (this one) so the README's links to CONTRIBUTING.md / AGENTS.md / CHANGELOG.md resolve when this lands.

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[otabek05]

Well done!