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>

go.mod

@@ -15,7 +15,6 @@ require github.com/kami/hexis v0.0.0
 require (
 	github.com/dustin/go-humanize v1.0.1 // indirect
 	github.com/google/uuid v1.6.0 // indirect
-	github.com/kami/praxis v0.0.0
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/ncruces/go-strftime v1.0.0 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
@@ -25,8 +24,4 @@ require (
 	modernc.org/memory v1.11.0 // indirect
 )
 
-replace github.com/kami/praxis v0.0.0 => /home/kami/apps/praxis
-
-replace github.com/kami/nexus v0.0.0 => /home/kami/apps/nexus
-
 replace github.com/kami/hexis v0.0.0 => /home/kami/apps/hexis