Docs: add project documentation foundation and architecture docs

[SolfE]

Summary

• add docs hub and documentation entrypoints
• add engineering decision records for major technical choices
• add architecture overview and core domain documents

Problem

The repository had little navigable documentation for structure, architectural context, and major engineering decisions.

Solution

Create a lightweight documentation hub, record key technical decisions in a blog-style format, and add architecture documents for the overall system plus three core domains.

Changes

• docs hub and templates
• decision records 001-009
• architecture overview, streak, word, and book domain docs

Related Issues

• None

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Replace workstation-specific links with repo-relative paths

The verification section is linking to an absolute local path (/Users/solfe/...) that only exists on the original author’s machine. In GitHub and in other developer clones this link is dead, so readers cannot open the referenced code and the decision record loses traceability. The same pattern appears across multiple new decision docs, so these links should be rewritten to repository-relative paths.

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