feat(blueprint): RepoConfig parity (max_budget_usd, memory_token_budget)

**Context:** ROADMAP.md → Blueprint RepoConfig parity

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## Component

CDK / infrastructure

## Describe the feature

Extend Blueprint construct to persist per-repo default **`max_budget_usd`** and **`memory_token_budget`** in DynamoDB. Orchestrator already merges `max_budget_usd` when present; hydration uses a fixed memory token cap today.

## Use case

Operators configure budgets in CDK Blueprint but memory cap and defaults are not fully persisted/merged—surprising behavior vs documentation.

## Proposed solution

1. Blueprint props → RepoTable fields for both values.
2. Orchestrator + hydration read merged config (task override > repo > blueprint default).
3. CLI `bgagent repos` shows effective limits.
4. Migration: backfill existing repos with platform defaults.
5. Tests: construct snapshot + handler unit tests.

## Other information

- Design context: `docs/design/REPO_ONBOARDING.md`.

- [ ] This might be a breaking change

Inizia da docs/design/REPO_ONBOARDING.md, quindi segui le Blueprint props attraverso RepoTable, l’orchestrator e gli hydration paths, oltre al punto di ingresso della CLI `bgagent repos`. Esamina i test construct snapshot e handler unit menzionati nell’issue; il lavoro è completato quando entrambi i limits persistono e vengono uniti con la precedenza indicata, i repos esistenti vengono popolati retroattivamente e i limits effettivi compaiono nella CLI.

Scritto dal modello di indicizzazione a partire dal testo della issue.