docs(cli): add Commonly CLI user guide

[lilyshen0722]

Summary

A single-page user guide for the `commonly` CLI. No code changes — this closes the gap between the ADRs (the "why") and the per-feature docs (`LOCAL_CLI_WRAPPER.md`, `WEBHOOK_SDK.md`, the "how" for one feature) by giving a top-level "what is the CLI and how do I use it" reference.

Structure

• Quick start — two-command flow for both the `claude` wrapper and the Python scaffolder
• Command reference — all subcommands grouped by concept (auth, agents local-CLI, agents webhook-SDK, agents shared, pods, dev)
• Configuration — the three on-disk files (`~/.commonly/config.json`, `tokens/`, `sessions/`) + env vars
• `--instance` symmetry — documents the PR fix(cli): resolve --instance symmetrically (saved key OR URL) #202 behaviour (key OR URL)
• Common workflows — attach claude, write a Python bot, tail a pod, test against local dev
• Troubleshooting — revoked token, `spawn ENOENT`, Cloudflare UA, etc.

Also

• `cli/README.md` — one-page pointer to the full guide + run-from-source + test instructions (no such file existed)
• `docs/README.md` index — adds a `cli/` directory row and a prominent Getting Started link

Test plan

• Every command + option verified against `cli/src/commands/*.js` at merge time
• All cross-doc links resolve (LOCAL_CLI_WRAPPER, WEBHOOK_SDK, ADR-004/005/006)
• CI green (docs-only; should be fast)

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