fix(project): refuse nested project creation and model embedded-asset error

[tejaskash]

Summary

Post-review follow-ups from #1809

• Modeled error for missing embedded assets — EmbeddedAssetSource.read now throws EmbeddedAssetNotFoundError (extends AgentCoreCLIError, internal source, assetPath in meta) instead of a bare Error.
• Refuse nested project creation — FsProjectManager.create now walks up from the working directory looking for the agentcore/agentcore.json project marker and throws a modeled NestedProjectError (user source) if one is found, instead of scaffolding a project inside an existing one.

The "child lifted to root" behavior seen with bun run start is a bun run artifact — bun chdirs to the package root before running the script, so the child was created next to root rather than inside it. A compiled/linked binary would have nested the child inside root. Either way, nesting a project inside a project would confuse project resolution, so create now refuses it explicitly.

Testing

• New test: creating a project from inside an existing project rejects with NestedProjectError.
• Updated test: missing embedded asset rejects with EmbeddedAssetNotFoundError.
• Full suite: 509 pass, 0 fail. tsc --noEmit and Prettier clean.
• Manual: project create --project-name child from inside a scaffolded root project prints Error: cannot create a project inside an existing AgentCore project (found .../root/agentcore/agentcore.json) and exits 1.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.86%. Comparing base (8e2cecf) to head (22afd02).

Additional details and impacted files

@@             Coverage Diff              @@
##           refactor    #1875      +/-   ##
============================================
+ Coverage     95.85%   95.86%   +0.01%     
============================================
  Files           198      198              
  Lines          9341     9368      +27     
============================================
+ Hits           8954     8981      +27     
  Misses          387      387              

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[aidandaly24]

This looks good to me. I think it's a good choice to keep the Errors inside the files that they are used until we see more use cases for them across the package.