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

Start with docs/design/REPO_ONBOARDING.md, then trace Blueprint props into RepoTable, the orchestrator and hydration paths, and the `bgagent repos` CLI entry point. Review the construct snapshot and handler unit tests mentioned in the issue; done means both limits persist and merge with the stated precedence, existing repos are backfilled, and effective limits appear in the CLI.

Written by the indexing model from the issue text.