Add a gitmark badge: git mark badge, then a verified badge endpoint

[melvincarvalho]

There's no way yet to show on a repo that it's gitmarked. This adds one, in two steps.

1. git mark badge

A new command that prints badge markdown for the current repo. It reads the origin remote, the current branch and blocktrails.json, and builds a shields.io dynamic JSON badge that counts states in the pushed trail file:

[![gitmarks](https://img.shields.io/badge/dynamic/json?url=https%3A%2F%2Fraw.githubusercontent.com%2F<owner>%2F<repo>%2F<branch>%2Fblocktrails.json&query=%24.states.length&label=gitmarks&suffix=%20%C2%B7%20<chain>&color=f7931a)](<link>)

• The badge renders as gitmarks | 1 · txbt4, and the count updates whenever blocktrails.json is pushed.
• It links to the latest mark's transaction on the chain's explorer (derived from CHAINS[chain].explorer). It could link to a trail page later.
• Tested against melvincarvalho/delivery-day, whose trail is on txbt4. The badge renders gitmarks: 1 · txbt4.
• It's only as current as the pushed file: it shows what the repo claims, not what the chain confirms. Step 2 fixes that.
• Non-GitHub remotes: print the markdown with a placeholder URL for the raw trail file, and say so.

2. A verified badge (later)

A small endpoint that does what git mark verify does: re-derive each address from pubkeyBase and states, and check the outputs on the chain's explorer. It returns shields.io endpoint JSON:

• verified 3/3 · txbt4, green
• 2/3 verified, red, when a mark is missing or doesn't match
• unconfirmed, amber, while the newest mark is still in the mempool

git mark badge --verified would print the markdown pointing at it. It needs hosting and some caching, so it can be a separate piece; step 1 doesn't depend on it.

Also

• A Badge section in the README showing the markdown, and a badge on this repo if it gets a trail of its own.
• Tests: the badge URL built from a fixture remote and trail, and the non-GitHub fallback.