feat(15): comprehensive README — install, config, every feature, examples

[jordanpartridge]

Summary

Closes #15 (README portion — a full GitHub Pages docs site can come later from the same source if/when we want it).

Expands the README from ~40 lines (title + bullet list + install snippet) to ~525 lines covering every shipping feature with real, copy-pasteable examples sourced from the actual facade signatures, builder methods, and config keys.

What's covered

• Setup — install, vendor:publish, minimum env vars
• Configuration — connections, bot middleware, slash commands
• REST API client — every facade resource (posts/channels/users/teams/files/reactions/bots/webhooks/commands/emoji/status/system) with usage
• Sending messages — fluent Message + Attachment builder, threading, fields
• Bot framework — handler registration (class/name/wildcard), Handler base class with reply/react/typing, #[Middleware] attribute, queueable handlers
• Middleware — built-in table (IgnoreBots, Dedup, RateLimit, ChannelFilter, AdminOnly), how to write custom
• Guards — RequiresRole, RequiresPermission, ChannelMember
• Slash commands — registration, handlers, in_channel/ephemeral responses
• Interactive messages — action handler registration, button payload shape, context.action convention
• Streaming replies — MattermostStreamingReply::create → append → finalize
• Notifications — toMattermost, MattermostMessage, broadcast driver
• Filament panel — plugin registration + page list
• Artisan commands — listen / post / health
• Testing — Mattermost::fake(), full assertion list, MattermostFixtures builders
• Multi-server — named connections, per-call selection
• Local development — docker compose + integration test suite

Test plan

• All examples reference real facade methods (@method block on src/Facades/Mattermost.php)
• Builder example matches src/Messages/Message.php and src/Messages/Attachment.php
• Middleware list matches src/Bot/Middleware/
• Guard list matches src/Bot/Middleware/Guards/
• Config snippet matches config/mattermost.php after feat: vendor:publish config — bootstrap onboarding #30
• php artisan vendor:publish --tag=mattermost-config referenced (per feat: vendor:publish config — bootstrap onboarding #30)
• Test fixture example uses the context.action shape (per fix(8): read interactive action_id from context, not top-level #36)

Closes #15

[lexi-chief-of-staff]

Ship it. Comprehensive docs upgrade — accurate to facades/config, great examples, closes #15 solidly.

• Examples match @method block [from: GitHubReadFile conduit-ui/mattermost:src/Facades/Mattermost.php].
• Config snippet aligns post-#30.
• docker-compose.yml ships as advertised [from: GitHubReadFile].

Nits inline.