chore: initial scaffold for latprobe CLI tool

Sets up project structure, working conventions (CLAUDE.md), README with
the full assignment and roadmap, seeded CHANGELOG, and a buildable Go
scaffold (Step 0) with flag parsing and stub probe types.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/usage/step-0-scaffold.md

@@ -0,0 +1,51 @@
+# Step 0 — Scaffold
+
+## What this step delivers
+
+A buildable Go binary (`latprobe`) that parses flags and URLs, prints usage
+when invoked without arguments, and exits cleanly. No measurement logic yet —
+the full implementation is wired in during Steps 1–4.
+
+## Build
+
+```sh
+cd go
+go build -o latprobe .
+```
+
+## Usage
+
+```sh
+# No arguments → prints usage and exits with code 1
+./latprobe
+
+# With a URL → scaffold message until Step 1 is implemented
+./latprobe https://example.com
+```
+
+Expected output (scaffold stage):
+
+```
+latprobe — measure per-phase HTTP request latency
+
+Usage:
+  latprobe [flags] <url> [url ...]
+
+Flags:
+  -n, --count int   Number of requests per URL (default 1)
+      --json        Output results as JSON instead of text
+  -h, --help        Show this help
+
+Examples:
+  latprobe https://example.com
+  latprobe -n 5 https://example.com https://www.google.com
+  latprobe --json https://example.com | jq .
+```
+
+## Available flags (accepted by the parser, not yet functional)
+
+| Flag | Short | Default | Description |
+|------|-------|---------|-------------|
+| `--count` | `-n` | `1` | Number of requests per URL |
+| `--json`  | —    | false | Output as JSON instead of text |
+| `--help`  | `-h` | —     | Show usage |