writ registers --store and reads it nowhere: status reports on the default store regardless #753

writ registers --store on its root and reads it nowhere. A user who points writ at another store gets a
report on the default one, with nothing said.

$ writ status --store /some/other/root
Layers:
  ...
Store: 0 run(s) folded          <- folded from the DEFAULT store, not the one named

Root cause

A flag registered on a root that no leaf consumes. cmd/writ/writ/root.go:49 calls
cli.AddOutputFlags(rootCmd, &outputOptions), so cobra advertises --filter, --jq, --output/-o, and
--store on every writ command's help. Consuming them is a separate act, and writ performs it once:

$ grep -rn outputOptions cmd/writ
cmd/writ/writ/config.go:160    cfg.JSONOutput = outputOptions.Format == "json"
cmd/writ/writ/commands.go:290  var outputOptions cli.SinkOptions
cmd/writ/writ/commands.go:302  pipeline, err := cli.BuildPipeline(outputOptions, cmd.OutOrStdout())   <- runVerify

outputOptions.Store is read by nothing. The same root cause produces
#754 -- writ accepts any --output value, because
the string never reaches FormatterByName -- which is the milder face of it: that one accepts wrong input,
this one substitutes a different data source.

Why this one is not cosmetic

--store only means something when someone calls SetStoreRoot, and that has exactly one caller in the
tree:

cmd/devlore-test/devloretest/commands.go:61

Yet writ genuinely uses the execution store: readback.go reads cli.TracesDir() (lines 228, 247) and
cli.GraphsDir() (line 500) to fold runs into its inventory. So the flag is not merely ignored -- writ reads
a different filesystem location than the one it was told to read, and reports the result as though it had
complied.

An absent flag errors and the user tries something else. A present flag that silently substitutes a
different data source produces a confident wrong answer.

Fix

writ resolves --store through cli.SetStoreRoot before any command that touches the store, restoring on
exit as devlore-test does. Every command reading TracesDir() or GraphsDir() is affected: status,
repo, and anything else routed through readback.

Verification

• writ status --store <tmp> reports on <tmp>, and reports zero runs when <tmp> is empty.
• A relative --store resolves the way devlore-test's does; OpenTree requires an absolute path.
• Deploying into a relocated store and reading it back reports what was deployed.