fix(vscode): bundled sidecar crashed on startup (ESM loaded as CommonJS)

[matze4u]

The bug (production-breaking)

Every packaged VSIX shipped a dead sidecar. The bundled
sidecar/server.js is copied from the ESM packages/core build, but it
lands outside that package with no module marker, so Node loads it as
CommonJS and the spawned process dies on the first import:

SyntaxError: Cannot use import statement outside a module
[sidecar] process exited (code=1, signal=null)

With the sidecar dead, every request (connect, chatSend, chatCancel)
times out: the index statusbar stays "Index: idle", chat send produces
nothing, and stop can't reach an already-crashed process — in both
API and CLI mode (it's upstream of the provider). The dev path worked
only because packages/core is type:module; resolveSidecarPath
prefers the bundled copy when present, so a stale bundle shadowed the
working dev sidecar.

Fix

bundle-sidecar.mjs now writes sidecar/package.json {"type":"module"}
next to the copied bundle → Node loads it as ESM. .vscodeignore keeps
sidecar/, so the marker ships in the VSIX.

How it was found

The SidecarClient piped the child's stderr but never read it, so the
crash was invisible — only a 120s client timeout showed. Second commit
forwards the child's stderr / spawn-error / exit to console.error
(Extension Host Developer Tools). That surfaced the ESM-load crash
immediately and is kept as a permanent diagnostic.

Verification

• Regenerated bundle boots: node clients/vscode/sidecar/server.js →
  agent core ready, answers connect with {ack:true,...}.
• Direct chatSend probe against the sidecar streams correctly
  ({token} frames + terminal done:true with usage) — confirming the
  ChatHandler / backend / client code were already correct; only the
  bundle's module type was wrong.
• clients/vscode typecheck + 40 vitest tests green.

Follow-up (not in scope)

In dev, resolveSidecarPath prefers the bundled copy over the live
packages/core/dist/server.js, so a stale bundle can shadow live core
edits. Worth preferring the dev path under extensionDevelopmentMode —
separate change.