Remove the internal forge hostname and add the doc manifest — this repo's main is un-gateable without them

[forkwright]

fix(docs): remove the internal forge hostname and add the doc manifest

Two pre-existing lint errors made this repo's main un-gateable, so no branch could
reach green regardless of its own quality. Found while gating unrelated work.

CONTRIBUTING.md carried an internal .lan hostname in three places -- the forge's
address written into tracked content. That address is per-box state, not a property
of this repository, so it is now referred to as with a note saying where a
reader gets the real value: their own git remote, or the kanon MCP binding the
session-start probe reports. Removing a fact without saying where it lives would just
move the problem.

This matters beyond the lint: an internal hostname in a tracked tree is exactly the
class of content that must not survive into a public repository, and this repo is in
scope for the fleet's public-flip work.

docs/MANIFEST.toml did not exist while docs/ did. Seven entries, one per document,
classified by reading each rather than by pattern: two decision records, four
authored, and the mesh config example. Three are marked non-evergreen with the reason
stated in place -- the lexicon calls itself a living registry, the reference-store
doc describes an arrangement built on a crate that is still planned, and the config
example is a snapshot of supported hardware.

The archive subtree is deliberately absent. Git is the archive, and a second index of
superseded documents is a copy of what history already guarantees.