repo set and unset replace add and remove, a clone takes git's name, and a group refuses a verb it does not have - #898

… it does not have

A layer has exactly one registration, so registering one that is registered
re-points it rather than failing: the verb is set, the vocabulary config set and
config unset already use for a keyed value. unset is idempotent. set to the root a
layer already has narrates unchanged. Every refusal precedes the clone, where add
cloned first and refused after. add, remove, rm and ls are gone, not aliased.

The record says what changed. source is the URL writ would fetch from -- the
upstream's remote, else origin, else empty -- and root is where writ reads; owner
says whose tree it is, and previous carries what was displaced or removed, so a
script reads on stdout what narration says on stderr. repo list carries source and
owner on every row.

A clone writ made goes with its registration. unset removes it and says so; a set
that re-points elsewhere removes the one it displaces; a tree registered by path
is never touched. --dry-run emits the record and does nothing. Removal passes a
guard that decides from the path, not from the record.

A group refuses a verb it does not have. Cobra checks unknown commands at the root
alone and abandons a runless group before validating its arguments, so writ repo
add exited 0 and printed help with add retired -- as did every unknown verb under
every group in writ, lore and star. Each program now validates the command line
before executing it, using cobra's exported Find and its own NoArgs, so the
message is cobra's by construction and exits 64 through the mapping §9 already
had. No group gains a RunE; invariant 7 stands. §3 states the rule.

Closes #791
Closes #792
Closes #897

… one

repo set <layer> <url> cloned into repos/<layer>: three directories named base,
team and personal, whatever repositories they held. The clone now takes the name
git clone would give it -- the last path component with a trailing slash and a
.git suffix stripped -- so noblefactor-ops.git lands at repos/noblefactor-ops and
the directory says which repository it holds. Two layers whose repositories share
a name are refused, naming both, before anything is cloned. A URL that yields no
name is refused as git refuses it. owner is unaffected: the clone is still under
the writ home.

The layer-journey scenario, skipped on develop for want of repo set, ran on this
pull request's first CI and asserted #793 in the same step as #791. The
scenario is the ruled interface; the code now follows it, and make test-scenario
is in the pre-flight, where the first run left it out.

Closes #793