dsh plugin exec cannot resolve installation peers before the first profile launch

[franksong2702]

DSH 0.1.2-rc.1 can leave a freshly installed plugin command unable to resolve peer packages supplied by the DSH installation.

Reproduction

1. Start with a fresh DSH home and DSH 0.1.2-rc.1.
2. Install an out-of-tree plugin that exposes a bin and declares @deepseek-ai/schemastery as a peer.
3. Before launching the profile, run the plugin bin through dsh plugin --profile <profile> exec <bin>.

The command fails with:

Error [ERR_MODULE_NOT_FOUND]: Cannot find package @deepseek-ai/schemastery imported from <profile>/node_modules/<plugin>/lib/...

A profile launch first makes the same command succeed.

Cause

Profile launch prepares $DSH_HOME/profiles/node_modules, the shared fallback containing installation-provided packages. The dsh plugin pnpm forwarder did not prepare that fallback, so a plugin executable invoked before the first launch could not resolve its DSH-provided peers.

Proposed fix

Prepare the installation module fallback after profile initialization and before forwarding every dsh plugin command. A self-contained built-bin regression installs a fixture with a Schemastery peer and executes it immediately.

Reference implementation: franksong2702@c90ea79

Verification

pnpm exec vitest run --config vitest.e2e.config.ts apps/cli/tests/built-bin.e2e.ts -t "anchors a relative add spec|makes installation peers available|activates a dependency that gained"
# 3 passed

pnpm run doc-sync
# 32 passed, 0 failed, 0 skipped

pnpm run build
# exit 0