Add a dbt-optimizer agent: scan dbt projects for fixable issues with cost/impact reporting

## Feature

Add a new native primary agent, `dbt-optimizer`, that scans an existing dbt project for concrete, fixable issues and proposes targeted fixes with cost and impact reporting. This is the agent behind the planned Optimize workflow.

**Input:** an existing dbt project (connected repository and environment).
**Output:** candidates (detected issues), fixes for user-selected candidates, impact reports (batch effect incl. estimated cost change), and PRs for applied fixes.

**Detection scope** (from the dbt-optimization taxonomy research, `docs/internal/2026-08-12-dbt-optimization-taxonomy-research.md`):
- materialization & incremental processing (strategy-aware: merge / delete+insert / insert_overwrite / microbatch, precondition-gated)
- warehouse physical design (clustering / partitioning / sort keys, query-history gated)
- SQL anti-patterns, DAG economics (fan-out, duplicate scans, dead models), run-level orchestration (Slim CI, threads, full-refresh overuse), tests/docs/storage

**Safety requirements:** scan phase is read-only; warehouse writes denied non-overridably; SQL rewrites gated on decidable equivalence; cost claims only from attributed query-history evidence.

Named `dbt-optimizer` (not `optimizer`) because more optimizer agents are planned later.

Start with docs/internal/2026-08-12-dbt-optimization-taxonomy-research.md and map its detection areas to the planned dbt-optimizer agent behind the Optimize workflow. Define the read-only scan, candidate selection, fix application, cost/impact reporting, and PR outputs, while preserving the stated warehouse-write, equivalence, and query-history safety gates. Done means the named agent covers the agreed scope without weakening those requirements.

Written by the indexing model from the issue text.