docs: dependency DAG automation + workflows (#200)

[flyingrobots]

Implements dependency DAG generation + automation, and documents contributor workflows.

Closes #200

Summary:

• Adds issue/milestone dependency DAG generator (config-driven) and committed DOT/SVG artifacts.
• Exposes generation via cargo xtask dags and Make targets; supports --snapshot-label none|rolling|YYYY-MM-DD.
• Adds a scheduled GitHub Action that refreshes DAG outputs via PR only when outputs change (snapshot labels omitted to avoid churn).
• Adds docs/workflows.md and links it from README + AGENTS.md.

Test Plan:

• cargo xtask dags --snapshot-label none
• cargo xtask dags --snapshot-label rolling --no-render

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📥 Commits

Reviewing files that changed from the base of the PR and between c893e50 and fa3d32a.

⛔ Files ignored due to path filters (5)

• Cargo.lock is excluded by !**/*.lock
• docs/assets/dags/issue-deps.dot is excluded by !**/*.dot
• docs/assets/dags/issue-deps.svg is excluded by !**/*.svg
• docs/assets/dags/milestone-deps.dot is excluded by !**/*.dot
• docs/assets/dags/milestone-deps.svg is excluded by !**/*.svg

📒 Files selected for processing (17)

• .cargo/config.toml
• .github/workflows/refresh-dependency-dags.yml
• .gitignore
• AGENTS.md
• Cargo.toml
• Makefile
• README.md
• docs/assets/dags/deps-config.json
• docs/decision-log.md
• docs/dependency-dags.md
• docs/docs-index.md
• docs/execution-plan.md
• docs/workflows.md
• scripts/generate-dependency-dags.js
• scripts/open_dependency_dags_pr.sh
• xtask/Cargo.toml
• xtask/src/main.rs

✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch echo/dependency-dags-automation

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[flyingrobots]

@coderabbitai review

Merged latest main into this PR branch (including the PR-comment extractor and current CI/tooling baseline). Merge conflicts were resolved by preserving both PR-hygiene and DAG automation entries in docs/execution-plan.md / docs/decision-log.md and by keeping both .cache and Playwright artifact ignores in .gitignore.

Please review current head and post any actionables.

[coderabbitai]

@flyingrobots: I'll review the current head of PR #201 after the merge from main. I'll examine the DAG automation implementation, the resolved merge conflicts in the documentation files, and the overall changes to ensure everything is consistent and actionable.

---

🧠 Learnings used

Learnt from: CR
Repo: flyingrobots/echo PR: 0
File: AGENTS.md:0-0
Timestamp: 2025-12-28T23:14:28.103Z
Learning: Keep `main` pristine. Feature work belongs on branches named `echo/<feature>` or `timeline/<experiment>`.

---

Learnt from: CR
Repo: flyingrobots/echo PR: 0
File: AGENTS.md:0-0
Timestamp: 2025-12-28T23:14:28.103Z
Learning: When a PR touches non-doc code, update `docs/execution-plan.md` and `docs/decision-log.md` in the same PR.

✅ Actions performed

Review triggered.

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