Documentation: README and wiki

[kicka5h]

Complete, accurate documentation for users and contributors.

Scope

README:

• Installation, configuration, all commands with examples, error table, project structure, CI/CD

Wiki (6 pages):

• `Home` — quick start and page index
• `Installation` — pipx/pip, verify, upgrade
• `Configuration` — named stashes, env var, INI format, priority order
• `Commands-Reference` — full reference for all 8 commands with flag tables
• `Writing-Good-Snippets` — best practices for self-contained, reusable snippets
• `Error-Reference` — every error message with cause and fix
• `Contributing` — dev setup, tests, release process

Acceptance criteria

• README covers all current commands including `snack stash` subcommands
• All wiki pages reflect the current config format (INI, named stashes)
• `Writing-Good-Snippets` page published to wiki
• No references to the old `stash=` config format as the recommended approach