feat(assets): add AssetManager and CDK scaffold templates

[tejaskash]

Summary

Introduces the asset embedding + rendering subsystem — the foundation for project scaffolding. AssetManager renders an asset tree from either Bun.embeddedFiles (compiled standalone binary) or the source filesystem (dev / node distribution), applying Handlebars templating with HTML escaping disabled and writing each file atomically.

This is PR 1 of 3 in a stacked series:

1. this PR — core: AssetManager, atomicWrite, CDK templates, tooling ignores
2. build: scripts/build.ts that bundles/embeds assets (stacks on this)
3. wiring: thread AssetManager into FsProjectManager (stacks on this)

What's included

• src/fs/atomicWrite — central temp-file + rename write util so a crashed render never leaves a half-written file behind
• src/assetManager — AssetManager, types, barrel, unit tests, snapshot baseline
  • source vs embedded modes selected by embeddedFiles.length
  • Handlebars noEscape (assets are code/config, not HTML)
  • path-traversal guards on every output path
  • gitignore.template / npmignore.template → .gitignore / .npmignore on render
• src/assets/cdk — CDK project scaffold templates (payload copied verbatim to generated projects, not CLI source)
• tooling: exclude src/assets from tsc, bun test, coverage, oxlint, prettier (the templates ship their own *.test.ts depending on packages the CLI doesn't install)
• add handlebars dependency

Testing

• bun run typecheck — clean
• bun test — 266 pass, 1 snapshot
• includes a byte-for-byte render snapshot of the full CDK tree

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.03%. Comparing base (56f62e7) to head (356105c).

Additional details and impacted files

@@             Coverage Diff              @@
##           refactor    #1809      +/-   ##
============================================
+ Coverage     93.94%   94.03%   +0.08%     
============================================
  Files           122      125       +3     
  Lines          6462     6555      +93     
============================================
+ Hits           6071     6164      +93     
  Misses          391      391              

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