docs: document uf init Step 6 UF-reference injection into speckit command templates

[yvonnedevlinrh]

What changed

uf init (Step 6 of uf.init.md) now injects Unbound Force references into generated speckit command templates:

• Spec-phase (speckit.specify.md, speckit.plan.md, speckit.tasks.md): .specify/memory/constitution.md awareness + Dewey (dewey_semantic_search).
• Implement (speckit.implement.md): constitution + Dewey + /uf.review-council before completing implementation.
• Constitution (speckit.constitution.md): constitution + Dewey.

Source: PR for opsx/add-speckit-ref-assertion-test (unbound-force/unbound-force#548 + unbound-force/unbound-force#549). Authoritative layer per ADR-002
(docs/decisions/002-uf-refs-source-of-truth-layer.md).

Pages needing updates

• docs/architecture.md — "What uf init Does" (~line 174).
• docs/usage.md — tool-owned files updated by uf init (~line 216).
• unbound-force/website — mirror the above.

Acceptance

• Docs describe which references uf init injects into which blocks.
• The implement-only scope of /uf.review-council is documented.