Consolidate module guides and unreleased purchasing documentation

[gdarko]

User-facing module guides still live mainly in package READMEs, while the purchasing guide lives in the application repository. Consolidate that material into the content-only docs repository without presenting unreleased app behavior as part of alpha.10.

• Add v3 guides for Tasks & Projects 0.2.1 and AI Assistant 1.0.0, with an Optional modules navigation group and links from the existing module/MCP guides.
• Record source review against the alpha.10 host extension runtime and pinned module commits. Correct the AI configuration steps to match the enabled-only connection-test control.
• Import the purchasing guide from the current app PR #903 head (9fba5b7a) into drafts/v3/guide/purchasing.md. It stays outside published books, search, and AI indexing until its release is reviewed.
• Document guide ownership, consolidation sources, and publication follow-ups. Engineering specifications and operational runbooks remain with their owning codebases.
• Preserve v2 content, both release baselines, existing URL/anchor compatibility, and screenshot provenance.

Validation: the canonical website validator passes with 71 documents, 107 assets, and 71 current source-review hashes. git diff --check passes. No new screenshots were introduced. Browser review of the new pages is pending because no browser is connected, so this PR is a draft.

Docs #31 has merged into master. The separate website renderer PR InvoiceShelf/website#54 was still open when checked; this change does not assume a production deployment or cutover. Cross-repository README link-only replacements can follow after the canonical guides land.