master: read the writable-volumes cap from master.toml (#12009)

* master: read the writable-volumes cap from master.toml

-maxWritableVolumes becomes the master.max_writable_volumes config key,
so the new master limit is set the same way as other master options
instead of a flag wired into every command that embeds a master.

* master: name the writable cap master.max_writable_volumes_per_layout

The cap applies per volume layout (collection + replica placement + ttl +
disk type), so the key names its scope instead of reading as a global cap.

* master: keep an embedder's MaxWritableVolumes when the key is unset

NewMasterServer overwrote option.MaxWritableVolumes with the viper
value, which reads as 0 whenever master.toml omits the key, silently
dropping a cap a Go caller supplied through MasterOption. Seed the
default from the option instead so the field survives an absent key;
an explicit config entry still wins.

* master: drop stray nested-checkout gitlinks

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

---------

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

weed/command/master_follower.go

@@ -47,7 +47,6 @@ func init() {
 	mf.metricsIntervalSec = aws.Int(0)
 	mf.raftResumeState = aws.Bool(false)
 	mf.maxParallelVacuumPerServer = aws.Int(1)
-	mf.maxWritableVolumes = aws.Int(0)
 	mf.vacuumIntervalSeconds = aws.Int(840)
 	mf.vacuumDeleteEmptyAfterSeconds = aws.Int(0)
 	mf.telemetryUrl = aws.String("https://telemetry.seaweedfs.com/api/collect")