docs(phase6): documentation site, landing page and open-source scaffolding

Material for MkDocs under docs/, covering self-hosting, the Rune, the
quick-add and filter languages, importing from Todoist, CalDAV,
mail-to-task, MCP, the AI layer and the API. The version is pinned: the
maintainers have announced that MkDocs 2.0 removes the plugin system
with no migration path, and an unpinned install would break this build
on the day it lands.

The landing page shows the quick-add parser as actual text rather than a
screenshot of it — that is the thing worth showing, and it reads better
as the real thing. It shares the app's design tokens, so the site and
what it describes look like one project.

MIT licence, contributing guide, security policy and issue templates.
Pages deployment is behind workflow_dispatch, so the docs build on every
push but nothing is published until the owner chooses to.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/filters.md

@@ -0,0 +1,65 @@
+# Saved filters
+
+A filter is a small expression describing a slice of your tasks. The grammar is
+Todoist's, so if you are coming from there you already know it.
+
+```
+today & p1
+#Firma & @regnskab
+overdue | (today & assigned to: me)
+7 days & !@venter
+```
+
+## What you can write
+
+| Term | Matches |
+|---|---|
+| `today`, `i dag` | Due today. |
+| `tomorrow`, `i morgen` | Due tomorrow. |
+| `overdue`, `forsinket` | Due before today. |
+| `7 days`, `14 dage`, `2 weeks` | Due between now and then. A window, not a single day. |
+| `no date`, `ingen dato` | No due date at all. |
+| `p1` … `p4` | That priority. |
+| `#Firma` | In that project. |
+| `@regnskab` | Carrying that label. |
+| `assigned to: me`, `tildelt: mig` | Assigned to you. |
+| `assigned to: anders@example.dk` | Assigned to that person. |
+| `assigned to: none` | Assigned to nobody. |
+| `recurring`, `gentagen` | Repeats. |
+| `completed`, `færdig` | Already done. |
+| `subtask`, `underopgave` | Has a parent. |
+| `2026-12-24` | Due on that exact day. |
+| anything else | Searched for in the title and description. |
+
+## Combining them
+
+| Operator | Means |
+|---|---|
+| `&` | Both. |
+| `|` or `,` | Either. |
+| `!` | Not. |
+| `( )` | Grouping. |
+
+`&` binds tighter than `|`, so `overdue | today & p1` means
+`overdue | (today & p1)`. Use brackets when you mean otherwise.
+
+## Some that are worth saving
+
+```
+today & p1                      What has to happen today
+overdue                         What slipped
+7 days & !@venter               The week, minus what you are blocked on
+#Firma & assigned to: me        Your part of a shared project
+p1 & no date                    Important, unscheduled — usually a mistake
+recurring & overdue             Habits you have stopped doing
+```
+
+## Why an expression is refused
+
+A filter is compiled before it is saved. A filter that cannot run is worse than
+none: you would find out later, from a list that is empty for a reason nobody can
+see. If you get an error, it names the part it could not read.
+
+!!! note "Labels are personal"
+    `@regnskab` matches *your* label of that name. Somebody else using the same
+    word on their own tasks is a different label, and your filter will not find it.