docs: add ECOSYSTEM.md and the README process-flow section

[Chheng64]

What

Two documentation additions on the same theme — telling a reader what the toolkit is and what it fits with, without moving detail out of the documents that already hold it.

• ECOSYSTEM.md — what "works with your existing workflow" means, stated structurally rather than as a partner list. Plus a README docs-table row and a CHANGELOG entry.
• A Process flow section in README.md — the whole run in one picture, between "What you'll build" and "Build your first project".

Why this shape

ECOSYSTEM.md

• Every artifact is markdown, HTML, JSON or CSV on disk; every check is a zero-dependency Node script with an exit code. Anything that reads a file or runs a command can participate — that claim carries the page, and each category backs itself with an existing document (FAQ.md, VALIDATION_ENGINE.md, ARTIFACT_FLOW.md, DIFFERENTIATORS.md, checks.yml).
• A Future connectors section quarantines the planned items — the dtk CLI, the shared artifact store, the Phase 5 packs — each row linking its PUBLIC_ROADMAP.md phase, so the line between "works today" and "planned" stays visible.
• Named ECOSYSTEM.md rather than INTEGRATIONS.md: the page is about compatibility, not built-in connectors. The toolkit ships none; that is the point, not a gap.

README process flow

• One Mermaid overview — brief → workflow → artifacts → validators and human gates → handoff → package — then the four claims that carry it, in existing repo language, a one-row comparison against prompt-based AI, and the twelve states in machine order with the three human gates marked.
• It summarizes and links; WORKFLOW_GUIDE.md and ARCHITECTURE.md keep the detail. Same division the landing-page rewrite established in docs: rewrite README as landing page, extract FAQ #3.
• STATE 10 · Revision is shown as the loop rather than a step, and STATE 12 before 11, because that is the machine order — a reader meeting the states for the first time should meet them true.

Validation

• node tools/linkcheck.mjs — 954/954 internal links resolve, 0 findings, exit 0
• node tools/mermaidcheck.mjs — 35 blocks across 89 files, 0 findings, exit 0

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