Gate prune's local-cleanup prose on --json and give make build a rule (closes #108)

CleanupLocalSnapshots wrote three prose lines to stdout with no --json
awareness, and they covered every branch of the function, so no input
avoided them: `vaultik prune --json | jq` failed even after the banner
fix. -q never helped either, because printlnStdout and stdoutf write
straight to Vaultik.Stdout and never consult Vaultik.UI, which is what
SetQuiet affects.

The issue offered three fixes and asked for a decision. Taken: thread
*PruneOptions into the function and gate each write on !opts.JSON,
matching PruneBlobs -- its sibling phase, which already takes the same
struct -- along with RemoveSnapshot and remote info, so the package has
one pattern rather than two. Rejected: moving the lines to log.Info,
because the logger's default level is slog.LevelWarn, so that would not
relocate them to stderr, it would delete them from a plain `vaultik
prune`, and the removal of rows from the local index is not something to
narrate only under --verbose. Also rejected: putting the stale-record
count into PruneBlobsResult, whose every field is blob-scoped and which
is produced by the later phase; a prune document covering both phases is
a reasonable thing to want, but that is a schema design question, not a
stream-hygiene fix. The two events are duplicated as log.Info records,
which PruneBlobs already does alongside its own prints, so they survive
on stderr under --verbose.

Also closes #110. `make build` printed "Nothing to be done for 'build'"
and exited 0 without producing a binary: build was listed in .PHONY with
no build: rule anywhere, and declaring a name phony is exactly what
converts make's "No rule to make target" error into a silent success.
Fixed with `build: vaultik`, keeping vaultik: as the file rule. Audited
all 19 .PHONY names: build was the only one without a rule, and vaultik
is correctly absent from .PHONY, being a real file target.

Tests, each verified to fail with the fix reverted rather than assumed
to. CleanupLocalSnapshots leaves stdout untouched under --json in all
three branches (stale records, none, empty index) and still emits every
line without it, so the guard cannot be satisfied by deleting the
output. prune --json runs end to end through Entry, cobra and fx over
the process's real stdout descriptor against a file:// store, asserting
exactly one JSON document, in both the stale and non-stale branches. And
a parse of the Makefile asserts every .PHONY name has a rule and that
build reaches the rule producing the binary, which keeps the audit true
for names added later; it is a parse rather than an invocation of make
because `make test` is what runs it, so shelling back into `make build`
would nest a build inside the test run. The property a parse cannot
establish -- that the recipe still fails when the build fails -- was
verified by hand against a deliberately broken tree: make build exits 2
and produces nothing.

cmd/vaultik gains its first test file, so `make test` now reports 16
packages ok where it reported 15. flagConfig and programName constants
are extracted in the CLI tests because the new argument vector pushed
"--config" and "vaultik" over goconst's threshold.

README's stdout/stderr section described the banner as "the other thing
that writes to stdout", which this defect contradicted; it now states
the contract that holds, which is that stdout under --json is the
document and nothing else, for prune as well as for the other four.

internal/cli/entry_banner_test.go

@@ -28,8 +28,13 @@ const (
 	cmdRemote   = "remote"
 	cmdInfo     = "info"
 
-	flagJSON  = "--json"
-	flagQuiet = "--quiet"
+	flagJSON   = "--json"
+	flagQuiet  = "--quiet"
+	flagConfig = "--config"
+
+	// programName is argv[0] as the real process receives it. Entry
+	// strips it before scanning, so it has to be present.
+	programName = "vaultik"
 
 	// someSnapshotID is any snapshot identifier: these tests never run
 	// the command, so it only has to occupy the positional argument.
@@ -65,7 +70,7 @@ var jsonArgumentVectors = map[string][]string{
 	// A --json invocation that also carries a flag with a value, so the
 	// scan cannot be fooled by an argument that consumes the next one.
 	"json with config": {
-		"--config", "/nonexistent/vaultik.yml", cmdSnapshot, cmdList, flagJSON,
+		flagConfig, "/nonexistent/vaultik.yml", cmdSnapshot, cmdList, flagJSON,
 	},
 }
 
@@ -222,7 +227,7 @@ func TestEntryJSONStdoutIsExactlyOneDocument(t *testing.T) {
 	})
 
 	os.Args = []string{
-		"vaultik", "--config", configPath, cmdSnapshot, cmdList, flagJSON,
+		programName, flagConfig, configPath, cmdSnapshot, cmdList, flagJSON,
 	}
 
 	stdout := captureProcessStdout(t, Entry)