feat: posts and snippets commands

[imjohnbo]

Part 3 of 8. Base: #15.

What this adds

Two endpoint families that the CLI did not reach at all.

Posts:

kit posts list [--include-content]
kit posts get <id>

Snippets:

kit snippets list [--snippet-type <inline|block>] [--archived] [--include-content]
kit snippets get <id>
kit snippets create <name> --type inline --content "..."
kit snippets create <name> --type block --html "..."
kit snippets update <id> [--name] [--content] [--html] [--archive] [--restore]

Design notes

An inline snippet stores Liquid text in content. A block snippet stores HTML in document_attributes.value_html. The create command checks the pairing before it sends. A mismatch reads as a CLI error instead of a 422.

Snippets have no delete endpoint. update therefore takes --archive and --restore.

Post detail output prints publication_id. A post and a broadcast that came from the same publication share this value. It is how you match one to the other.

Review notes

Issue #7 reported description changes on the three snippet endpoints. That was the signal to check coverage. The CLI had none, so this branch adds it.

Tests: 198 before, 232 after.

Closes #5
Part of #7
Part of #12

---

The stack

Merge from the top down. Each part builds on the one above it.

	PR	Contents
1	#14	Client query support and shared helpers
2	#15	Sequence CRUD and sequence emails
3	#16 (this one)	Posts and snippets
4	#17	Broadcast filters, account-wide stats, link clicks
5	#18	Subscriber slim listing, warnings, filter
6	#19	Tag rename, remove-by-email, bulk delete
7	#20	Account endpoints and purchase create
8	#21	Spec coverage map and its tests

Base branch: main for part 1, the previous part's branch for the rest.
GitHub retargets each base to main as the part below it merges.