chore(cli): decouple cdktn bin entry point from build output

[X-Guardian]

Description

Two changes to how cdktn-cli builds. The CLI launcher moves out of the build output into a committed bin/cdktn, and the esbuild build config compiles into a gitignored build-config/ directory instead of next to its source. No user-facing CLI behaviour changes — same cdktn command, same flags.

Why

Previously the bin field pointed at bundle/bin/cdktn, a file that only exists after build.ts copies it in (fs.copySync("src/bin/cdktn", "./bundle/bin/cdktn")). That means the bin target is absent until a build runs. yarn 1 tolerates this, but stricter package managers validate bin targets when linking workspace packages, so a bin pointing at a not-yet-built path breaks install. Committing a stable top-level launcher that lazily requires the bundle makes the bin target always present — before any build — and removes a build-time copy step.

This is a prerequisite for #170.

Approach:

1. New committed launcher packages/cdktn-cli/bin/cdktn — #!/usr/bin/env node + require("../bundle/bin/cdktn.js"). The bin field in package.json now points here (bin/cdktn) instead of bundle/bin/cdktn.
2. Delete src/bin/cdktn and drop the fs.copySync("src/bin/cdktn", "./bundle/bin/cdktn") step from build.ts — the launcher is no longer generated as build output.
3. Compile the build config to build-config/: compile-build-config now runs tsc build.ts --outDir build-config, and postbuild / watch invoke node build-config/build.js. Keeps a stray compiled build.js out of the package root; build-config is added to .gitignore.
4. Tests resolve the bin from package.json: convert.test.ts and init.test.ts now read cdktnCliPkg.bin.cdktn and resolve it against the package root, rather than hardcoding bundle/bin/cdktn — one source of truth that tracks the bin field.
5. Type let t: number in the rebuild-log plugin: also a consequence of (3) — compiling build.ts now type-checks it, and the bare let t; left t implicitly any. The annotation makes Date.now() - t well-typed.
6. Add build.ts to knip's cdktn-cli entry list: also a consequence of (3) — knip can no longer trace esbuild through postbuild (now a gitignored build-config/build.js) and flags it unused. Listing build.ts makes knip parse the import * as esbuild directly.

Checklist

• I have updated the PR title to match CDKTN's style guide
• I have run the linter on my code locally
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation if applicable
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works if applicable
• New and existing unit tests pass locally with my changes