chore: migrate to pnpm workspaces

[pyramation]

Summary

Replaces yarn 1 + symlink-workspace with pnpm workspaces, matching the setup used in the other monorepos (pnpm-workspace.yaml, pnpm -r scripts, lerna with npmClient: pnpm).

Root:

• workspaces field → pnpm-workspace.yaml; packageManager: [email protected]; yarn.lock → pnpm-lock.yaml
• scripts are now pnpm -r run <task> (plus deps: pnpm up -r -i -L); dropped symlink-workspace and the postinstall: yarn symlink hook (pnpm links workspace packages itself)
• lerna ^6 → ^8.2.4 with npmClient: "pnpm" / npmClientArgs: ["--frozen-lockfile"]

Packages:

• internal deps use workspace:* (@cosmwasm/ts-codegen-ast, @cosmwasm/ts-codegen-types)
• prepare: npm run build → prepublishOnly (pnpm runs prepare on every install, which caused a build during install)
• build scripts use && instead of ; so a failing tsc fails the build
• deps that only resolved through yarn's hoisting are now declared explicitly: @babel/parser + @types/{babel__generator,babel__traverse,minimist} in ts-codegen, @babel/generator/@types/babel__generator/glob in ast, @eslint/js + @eslint/eslintrc at the root (the flat eslint config imports them)
• packages/ast/test-utils/utils.ts imported RenderOptions from the built @cosmwasm/ts-codegen-ast/types path; now imports from ../src/types

CI (lint.yml, autofix.yml, run-tests.yaml): pnpm/action-setup@v4, cache: 'pnpm', pnpm install --frozen-lockfile, and per-package tests via pnpm --filter <pkg> test instead of cd <dir> && yarn test.

Docs/README development commands updated to pnpm install / pnpm build / pnpm test:*.

Source formatting churn in context.ts, imports.ts, bundler.ts, plugin-base.ts, types.ts is pnpm format output under prettier 3.9 (the old lockfile pinned 3.6) — autofix would produce it on the next run anyway.

Verified with a clean pnpm install --frozen-lockfile: build, 186 tests across both packages, and lint all pass.

Link to Devin session: https://app.devin.ai/sessions/bb89d57636c84c7383305191a303b1d4
Open in Devin Desktop: https://app.devin.ai/desktop/session/bb89d57636c84c7383305191a303b1d4?variant=devin
Requested by: @pyramation

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