docs: add Mermaid diagrams for better readability

[LSX-s-Software]

What

Replace selected ASCII architecture and lifecycle diagrams in the mdBook documentation with Mermaid diagrams, and enable Mermaid rendering in the documentation build.

Why

The architecture, sandbox lifecycle, egress-rule, and multi-node control-plane diagrams are easier to read, maintain, and render consistently as Mermaid diagrams than as fixed-width ASCII art.

Related issue

N/A

Scope and non-goals

• Convert relationship diagrams in the concepts and internals documentation.
• Add the mdBook Mermaid preprocessor and browser-side Mermaid initialization.
• Keep source directory trees, command examples, and other non-diagram text unchanged.
• No runtime, API, storage, scheduler, or generated-code behavior changes.

Design and behavior changes

• docs/book.toml enables the mdbook-mermaid preprocessor and loads mermaid-init.js.
• Mermaid diagrams preserve the original component relationships, labels, lifecycle transitions, storage paths, and control-plane routing semantics.
• The browser initializer loads Mermaid 11.12.0 from jsDelivr with an integrity hash and selects a light or dark Mermaid theme based on the mdBook theme.
• The distributed control-plane diagram shows Gateway as the data-plane proxy and Scheduler as the node selection/lookup service.

Compatibility and operations

• Public API or generated protocol: N/A; documentation-only change.
• Configuration or defaults: N/A.
• Snapshot manifest, artifact layout, or storage format: N/A.
• Upgrade and rollback: Reverting this commit restores the previous ASCII diagrams and removes Mermaid preprocessing.
• Host requirements, permissions, ports, or dependencies: Documentation builds now require mdbook-mermaid; CI installs mdbook-mermaid@0.17.0.

Validation

• make fmt
• make clippy
• make test-unit
• Relevant Rust integration tests
• make -C services test (required when services/ changes)
• Generated clients/server regenerated with the documented make target
• Documentation updated
• Benchmarks or performance comparison completed

Commands and results:

mdbook build docs
INFO Book building has started
INFO Running the html backend
INFO HTML book written to /home/ubuntu/AgentENV/docs/book

git diff --check
Passed

Skipped checks and reasons:

Rust, services, generated-code, and benchmark checks were skipped because this PR only changes documentation and documentation build assets.

Risks and reviewer notes

• Mermaid rendering depends on the pinned jsDelivr asset and the mdbook-mermaid preprocessor.

Checklist

• The PR contains one coherent change and no unrelated formatting or refactoring.
• New behavior is covered by tests, or I explained why testing is impractical.
• Logs and examples contain no credentials, tokens, or private registry information.
• I did not manually edit generated code without updating its source and regenerating it.

[github-actions]

🔍 OpenCodeReview found 2 issue(s) in this PR.

• ✅ Successfully posted inline: 2 comment(s)

[github-actions]

This newly added third-party action is pinned only to the mutable v2 tag. Pin it to a full commit SHA (while retaining the version in a comment) so a future tag movement or repository compromise cannot alter the documentation build.

Suggestion:

Suggested change

	- uses: taiki-e/install-action@v2
	with:
	tool: mdbook-mermaid@0.17.0
	- uses: taiki-e/install-action@<full-commit-sha> # v2
	with:
	tool: mdbook-mermaid@0.17.0

[github-actions]

This newly added third-party action is referenced by the mutable v2 tag. A tag can be moved or compromised, causing an unreviewed binary to run in the documentation build; pin taiki-e/install-action to a full commit SHA (and retain the version in a comment if desired).

[github-actions]

mermaid-init.js looks up mdbook-theme-* element IDs, but mdBook's built-in theme links use theme-* (for example, theme-light, theme-coal, and theme-ayu). Consequently none of these listeners are attached, so switching between light and dark themes does not reload/re-render Mermaid diagrams and their colors can remain from the initial theme. Use the actual mdBook theme IDs (or listen to the theme-change mechanism) before enabling this script here.