docs: give witan-core its own page

[blarghmatey]

witan-core was the one third of the documented trio with no page. It appeared
only as a row in an architecture table and a line in the package list, so a
reader wanting to know what it provides had to go read its README in the repo —
which is the gap the site exists to close.

Written as explanation rather than generated API reference, deliberately.
witan-core describes itself as "shared core for the witan MCP servers": it is an
internal dependency, not a library third parties build against, and a
symbol-by-symbol reference would add a large low-traffic section while implying
a stability promise the package does not make. It would also mean adopting
mkdocstrings, giving up the site's current no-plugin property — the thing
keeping a fall back to mkdocs-material cheap while Zensical is pre-1.0.

What the page covers is what a contributor actually needs:

• Why it exists. Not "shared code" in the abstract, but the specific class
  of code where two copies drifting is a correctness bug: the repo-key
  canonicaliser, whose output is the join key between the memory and code
  graphs, and the pinned omnigraph binary version, where a mismatch is a
  strict-format error rather than an inconsistency.
• The leaf invariant. witan_core imports neither server, which is what keeps
  the optional witan → witan-code mount a DAG instead of a cycle.
• Why the base package is stdlib-only, and why observability and sentry
  are separate extras rather than one.
• The version-floor trap, which is the thing that will actually bite a
  contributor. The uv workspace resolves witan-core by path, so adding a
  witan_core symbol and using it in a server passes locally and in CI, then
  fails at pip install time for anyone resolving from PyPI. Nothing automated
  catches it — the path resolution is precisely what hides it — and the pin
  comments record it having happened five times in witan-council and three in
  witan-code.

Co-Authored-By: Claude Opus 5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_017oVnpzjRJQVWq9hJGE2Rs7

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[Copilot]

Pull request overview

Adds a dedicated explanation page for witan-core, covering its architecture, dependencies, shared responsibilities, and version-floor maintenance risk.

Changes:

• Documents witan-core design and contributor guidance.
• Links the page from site navigation, indexes, and package listings.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
zensical.toml	Adds the page to navigation.
docs/index.md	Links witan-core to its explanation.
docs/explanation/witan-core.md	Adds the explanation page.
docs/explanation/index.md	Adds the page to the explanation index.

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