fix(cli): hard-exit on Node 25.x instead of soft warning + crash (#149)

The Node 25.x V8 turboshaft WASM JIT Zone allocator bug
(https://github.com/colbymchenry/codegraph/issues/81) reliably crashes
CodeGraph mid-indexing with `Fatal process out of memory: Zone` when
tree-sitter grammars get JIT-compiled. We already had:

- `engines: "node": ">=18.0.0 <25.0.0"` in package.json
- Lazy grammar loading (#61)
- A startup `console.warn` when Node 25+ is detected

But the recurring duplicates (#54, #81, #140, plus comments from
multiple unique users) show those defenses aren't enough:

- npm `engines` is a soft warning by default, so `npm install -g`
  doesn't block.
- The startup `console.warn` is a single yellow line that scrolls
  off-screen before the OOM 30 seconds later, so users connect the
  crash to "CodeGraph is broken" rather than "I'm on the wrong Node
  version" and file a fresh issue.

This patch turns the soft warning into a hard exit. On Node 25+ we
print a bordered banner that names the V8 root cause, embeds the
detected version, gives Node 22 LTS install commands (nvm + Homebrew),
and links to #81 — then exit(1) BEFORE any tree-sitter import
triggers WASM JIT. The previous behaviour is preserved behind
`CODEGRAPH_ALLOW_UNSAFE_NODE=1` for anyone who patched V8 themselves
or wants to test a future Node 25 fix.

The banner builder is extracted to `src/bin/node-version-check.ts` so
the test can import it without triggering CLI bootstrap. Five unit
tests pin the version interpolation, root-cause explanation, recovery
commands (nvm + brew), override env var, and #81 link — these are
load-bearing and shouldn't get edited away silently.

Suite: 509 → 514, all passing. Verified both paths manually by
flipping the threshold to 22 in dist and running on Node 22.20.0:
without the env var the CLI prints the banner and exits 1; with
`CODEGRAPH_ALLOW_UNSAFE_NODE=1` it prints the banner and continues.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>