Restructure Codra into npm workspaces and extract the review engine into @codra/core

[devarshishimpi]

Description

Converts the repo into an npm workspace monorepo. Moves shared contracts and the review engine out of src/ into real packages (packages/schema, packages/core), with the worker wired to them via ports and adapters.

What changed

1. Workspace structure

• Root package.json adds workspaces: ["packages/*", "apps/*"] and fan-out scripts.
• tsconfig.base.json shared by all packages.
• apps/worker holds the Cloudflare Worker entrypoint.
• CONTRIBUTING.md documents the new layout.

2. @codra/schema

• src/shared/* moved to packages/schema (types, zod contracts, only depends on zod).
• All @shared/schema imports updated to @codra/schema.

3. @codra/core

• Review engine moved to packages/core (pipeline, parsing, diffing, rules, prompts, etc).
• Platform coupling replaced with ports (@codra/core/ports); src/server/adapters/ implements them.
• packages/core/test/in-memory.ts lets the package's test suite run with no database, the acceptance bar for the extraction.
• picomatch moved from root deps to @codra/core.

Guardrails

• scripts/check-core-boundary.mjs (npm run check:boundaries) asserts @codra/core
  purity as a check rather than a convention: no hono/postgres/wrangler/Octokit in the
  manifest, no banned import specifiers, and no banned identifiers (AppBindings,
  KVNamespace, GitHubService, ModelService, …). The identifier half exists because a
  import type { AppBindings } leaves no runtime trace and would silently reintroduce the
  coupling this extraction removes.
• ESLint gains import-x/no-restricted-paths rules covering the new package boundaries.

Part of #66
Closes #67 #68 #69

Type of change

• Documentation update
• Chore (refactoring, dependency updates, etc.)

How Has This Been Tested?

• Unit Tests
• Integration Tests

Checklist:

• I have starred Codra on GitHub
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes
• I have signed the CLA

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