Project deploy: same project name + target collides on one CloudFormation stack

## Summary
Two separate projects that share the same `name` (in `agentcore.json`) and target, deployed to the same account+region, resolve to the **same** CloudFormation stack. The second deploy updates/overwrites the first project's stack instead of creating its own.

## Repro
1. Create two projects in different directories, both `name: "demo"`.
2. `agentcore deploy --target default` from each, same account+region.
3. Both map to stack `AgentCore-demo-default`; the second deploy overwrites the first's stack.

## Root cause
Stack identity is derived solely from `(project name, target name)` — `AgentCore--` (`src/assets/cdk/bin/cdk.ts` `toStackName`). The project spec has no stable unique id distinct from the human-chosen `name`, and the `agentcore:project-name` tag is just the name, so nothing disambiguates two same-named projects. The deploy path only verifies the account matches the target (`src/core/project/backends/cdk.ts`), not project identity.

## Impact
Silent overwrite of another project's deployed resources when names collide in a shared account+region.

## Suggested fix
Generate a stable project id at `project init`, store it committed in `agentcore.json`, stamp it as a stack tag (e.g. `agentcore:project-id`), and refuse to deploy when an existing stack of that name carries a different id.