docs: remove decisions log and clean references

[sandover]

Motivation

• The project should treat docs/record/ as the single canonical source of current design rather than retaining an ADR/decisions history in the repo.
• Remove the on-disk decisions log and update docs map and wording so indexes and guidance reflect the canonical docs-only policy.

Description

• Deleted the docs/decisions/ directory and all ADR files within it.
• Removed the docs/decisions entry from the docs map in AGENTS.md and updated docs/README.md to no longer reference the ADR/decisions section.
• Updated docs/record/README.md wording to reference the full docs index (excluding decisions) and adjusted docs/record/vision.md to say “canonical docs of record” instead of ADRs.

Testing

• Ran cargo fmt --all and it completed successfully.
• Ran cargo clippy --all-targets -- -D warnings and the check completed successfully without warnings.
• Attempted to run just ci-fast but it failed in this environment because just is not installed (error: bash: command not found: just).

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Codex Task