Add squash and collection commands

[HuiJun]

Summary

Extends the CLI toward feature parity with layer1-service by adding two
recently-added server capabilities:

• squash — squash the linear commit path between two locks into a
  single diff on the newer lock's commit (POST /orgs/{org}/repos/{repo}/squash).
• collection — manage Collections, layer1's lightweight groupings
  of refs (branches/locks/scratches) that can be read and queried as a
  union graph (/orgs/{org}/collections).

Commands

squash

flexo squash
<dstLock> must be the newer commit. Accepts lock names (resolved to
lock IRIs under the current org/repo) or full IRIs. Prints the resulting
squashed commit id.

collection

flexo collection list
flexo collection get
flexo collection create --ref --ref ...
flexo collection query (-q | -i ) -o

• create takes one or more --ref values; each is resolved to a branch
  IRI under the current org/repo (or passed through if already a full IRI).
• query runs SPARQL across the union of the collection's collected ref
  graphs; reads the query from -q/--query or -i/--input, writes
  results to -o/--output or stdout.

Implementation

• FlexoMmsClient: squash, listCollections, getCollection,
  createCollection, getCollectionModel, queryCollection, plus RDF
  parsing of collections (parseCollectionsFromRdf).
• New Collection model POJO (id, name, etag, collectedRefs).
• New SquashCommand and CollectionCommand (with list/get/create/
  query subcommands), registered in FlexoCLI + help listing.
• Ref resolution against server root context: refs are resolved
  against the server's configured root context (e.g. http://layer1-service),
  not the client base URL, since layer1 stores and validates refs under
  that root. getServerRootContext() derives it from an existing branch's
  commit IRI; absolute http(s) IRIs pass through unchanged.

Follows existing command/client patterns (BaseCommand, createClient,
ConsoleUtil, Turtle bodies, extractCommitId).

Testing

• ./gradlew clean test — green.
• New FlexoMmsClient tests: squash success/error, collection
  list/get/create (incl. empty-refs guard)/query success/error, and
  ref-resolution (server root context + absolute passthrough).
• Constructor smoke tests for both commands (matches existing command
  test style).
• Verified live against a locally-built layer1-service image (collections
  feature): collection create --ref master --ref feature-a resolves to
  http://layer1-service/... IRIs and creates; list/get round-trip;
  query returns the correct union count. squash reaches the server with
  the correct path/body (verified via the server's precise
  "lock does not exist" response).

Notes / limitations

• squash end-to-end is not exercised here because the CLI has no lock
  command yet (can't create the two locks squash operates on). The squash
  client method is unit-tested and the request path/body are verified
  against the live server.
• collection query returns HTTP 500 when two collected refs point to
  the same commit — a server-side union-graph construction issue
  (duplicate FROM/FROM NAMED), not a CLI bug; the CLI sends a valid query.
• Requires a layer1-service build that includes the collections feature.