fix(market): give every install staging tree its own pnpm workspace

[elkaix]

Problem

installGeneration writes a pnpm-workspace.yaml into the staging workspace only when build approvals exist to forward. Without one, pnpm resolves the workspace root by walking the ancestor chain — and when a directory above the staging tree is itself a pnpm workspace (a home folder used as one is the usual case), that workspace claims the install together with its store. Its node_modules/.modules.yaml records the store of a different pnpm major than the bundled one, so pnpm exits with ERR_PNPM_UNEXPECTED_STORE in a few hundred milliseconds and the market reports the plugin as unbuildable.

Reproduction on macOS (~/pnpm-workspace.yaml + ~/node_modules linked to store v11, packageManager: pnpm@11, bundled pnpm 10.34.5):

staging tree	result
in /tmp, no workspace file	install succeeds
under $HOME, no workspace file	ERR_PNPM_UNEXPECTED_STORE (v11 vs v10), exit 1 in ~234 ms
under $HOME, with pnpm-workspace.yaml	install succeeds

dshmarket classifies the failure and advises relinking the profile's node_modules — which never mismatched, so the advice cannot fix it.

Fix

• stageBuildApprovals always writes the workspace file now. With no approvals it is a minimal manifest (packages: ['.']) that pins the workspace root to the staging tree itself.
• storeMismatch() recognizes the failure by its code and extracts both named stores; a mismatch that survives triggers exactly one retry in a fresh staging workspace (disposable by construction — a failed install already discards staging, so the retry cannot touch anything outside it).
• The terminal failure detail names both stores and the ancestor-workspace cause instead of a bare pnpm code line.

Testing

End to end with the app's bundled node+pnpm against the real registry flow (installGeneration imported from the patched file):

• install with no ancestor workspace: ok: true
• simulated first-attempt store mismatch: retry engages (2 pnpm calls, traced) and succeeds
• persistent mismatch: ok: false with the actionable detail naming both stores
• storeMismatch() returns undefined for unrelated failures
• node --check clean; diff is confined to packages/dsh-desktop-market-installer/generations/installer.mjs