feat(cli): implement build --from <git-url> clone + stack detect

[pxivory-max]

Summary

• Replace the build --from stub for git URLs with real behavior: shallow-clone → stack detect → structured summary → cleanup.
• Add --keep-clone flag to retain the cloned repo directory after build.
• Export a detectStack() helper that reads package.json, pyproject.toml, Cargo.toml, or go.mod at repo root and returns runtime + package manager + project name.
• Add vitest unit tests covering Node (npm/pnpm/yarn/packageManager field), Python (pip/poetry), Rust, Go, empty dirs, and multi-manifest precedence.
• Other --from input kinds (url/path/doc) remain stubs — handled in follow-up PRs.

Relates to: #133
This is PR #1 in a series of small, focused PRs implementing the TODO stubs across the CLI.

Changes

packages/cli/src/commands/build.ts

• detectStack(dir) — reads manifest files, returns { runtime, packageManager, projectName }
• cloneAndDetect(input) — shallow-clones a git repo into $TMPDIR/sh1pt-build-<name>-<rand>, runs detectStack, returns clone path + result
• buildCmd action for kind === 'git': clone, detect, print summary, cleanup (or keep with --keep-clone)

packages/cli/src/commands/build.test.ts (new)

• 10 unit tests for detectStack using temp directories with fixture manifest files

Test plan

• pnpm test — all 1760 tests pass (632 files), including 10 new build tests
• pnpm typecheck — no new errors (pre-existing scale.ts duplicate identifier issue unchanged)
• pnpm lint — no lint scripts configured, clean pass
• Manual: sh1pt build --from https://github.com/profullstack/sh1pt prints build summary
• Manual: sh1pt build --from https://github.com/profullstack/sh1pt --keep-clone retains clone dir

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