GLSP-1636: Rework cross-repo linking and finish the pnpm-only CLI

[tortmayr]

What it does

Reworks glsp repo link onto pnpm and removes the remaining lerna/yarn-era code paths from the CLI after the pnpm migration.

• Rework repo link/unlink to inject link: overrides into each consumer's pnpm-workspace.yaml (dependency-ordered), sharing the singleton peers (sprotty, sprotty-protocol, vscode-jsonrpc, inversify) from glsp-client so they resolve to a single instance across the link.
• Build each linked repo so the overrides resolve to compiled lib/ output instead of empty source dirs (pnpm does not run yarn's prepare-on-install); --no-build skips it, --electron picks the Theia target.
• Do not link reflect-metadata: v0.2+ shares one registry on the global Reflect so multiple copies interoperate, and overriding it broke the linked build's types: ['reflect-metadata'] resolution (TS2688).
• Support linking glsp-eclipse-integration, whose pnpm workspace lives in a client/ subdirectory; the link build covers only the npm side and leaves the Maven server to a separate build.
• Remove lerna/yarn-era leftovers: the lerna publish fallback, the lerna.json version update, the yarn updateNext path and the PackageManager auto-detection; align the releng e2e specs and CLI README with the pnpm reality.

Part of: #1636

How to test

pnpm install && pnpm build && pnpm test    # 395 unit tests

Cross-repo linking (requires sibling clones; pick a preset):

glsp repo clone --preset theia    # or vscode / eclipse
glsp repo link  --preset theia    # clones are linked + built in dependency order
glsp repo unlink --preset theia   # restores pnpm-workspace.yaml byte-for-byte

Verified locally:

• pnpm build + 395 unit tests green
• theia / vscode / eclipse presets: clone → link → build → unlink round-trip clean
• Theia app runs against the linked stack (pnpm browser start); the GLSP server launches from the linked source
• inversify resolves to a single instance across all linked boundaries; reflect-metadata is not overridden
• eclipse links via its client/ workspace with no Maven invocation

Follow-ups

• A glsp repo watch command (running tsc -b -w across the linked upstream repos) would give live auto-pickup of source changes; investigated but not implemented here.
• The glsp-eclipse-integration and glsp-vscode-integration pnpm migrations are still on unmerged branches; linking those presets requires them on the default branch.

Changelog

• This PR should be mentioned in the changelog
• This PR introduces a breaking change (if yes, provide more details below for the changelog and the migration guide)