build: make go mod tidy runnable, and drop two dead requirements (V-454)

The vendored toolchain lives inside the module tree, so `go mod tidy` walked
Go's own compiler-error fixtures and died on files that are malformed on
purpose ("unicode//utf8": double slash). A nested module is not part of its
parent, so deps/go.mod ends the walk in three lines. deps/ is gitignored, so
the sentinel is generated by `make deps-sentinel`, which deps-go and deps now
depend on.

The tidy it makes possible drops github.com/kami/praxis, which no file
imports — Praxis is reached over HTTP, by contract. Its replace directive and
the unused nexus one went with it, so a build no longer expects two sibling
checkouts that nothing reads. vendor/ is committed, so `make tidy` re-vendors
in the same breath: a tidy alone leaves the next build failing on
"inconsistent vendoring".

Not wired into `make test`. A build target that rewrites go.mod is a surprise.

Verified: `make build` produces all 9 binaries and `make test` is green.

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

vendor/modules.txt

@@ -15,8 +15,6 @@ github.com/google/uuid
 # github.com/kami/hexis v0.0.0 => /home/kami/apps/hexis
 ## explicit; go 1.25.5
 github.com/kami/hexis/pkg/client
-# github.com/kami/praxis v0.0.0 => /home/kami/apps/praxis
-## explicit; go 1.23
 # github.com/mattn/go-isatty v0.0.20
 ## explicit; go 1.15
 github.com/mattn/go-isatty
@@ -79,4 +77,3 @@ modernc.org/memory
 modernc.org/sqlite
 modernc.org/sqlite/lib
 modernc.org/sqlite/vtab
-# github.com/kami/nexus v0.0.0 => /home/kami/apps/nexus