feat(pipeline): version tag on indexed nodes + diff_versions MCP tool

[isc-tdyar]

Adds version tagging on all indexed nodes and a diff_versions MCP tool for cross-version change detection.

Closes #1062. Depends on #467 (ObjectScript language support) merging first — the Storage node inline-JSON change targets ObjectScript Storage nodes.

New API

cbm_pipeline_set_version(cbm_pipeline_t *p, const char *version_tag)

Sets a version label (e.g. "28.0") on a pipeline instance. When set, the tag is written as a "version" property on every node. Also bypasses the incremental-index path so versioned indexing always does a full pass — the graph accumulates both versions side-by-side.

index_repository tool: new version parameter

{ "repo_path": "/repos/myproject-28.0", "version": "28.0" }

When omitted, a version-like segment (digits and dots) is auto-derived from the repo path. No behavior change for projects that don't use versioning.

diff_versions MCP tool

{
  "project":      "myproject",
  "from_version": "28.0",
  "to_version":   "30.0",
  "label":        "Class"
}

Returns {"added": [...], "removed": [...], "from_version": "28.0", "to_version": "30.0"}. Default label Class, limit 500 per side.

Files changed

• src/pipeline/pipeline.h — cbm_pipeline_set_version() declaration
• src/pipeline/pipeline_internal.h — version_tag field on ctx
• src/pipeline/pipeline.c — setter, free, skip-incremental guard, ctx wiring
• src/pipeline/pass_definitions.c — version property on all nodes; Storage nodes with structured docstring have fields merged inline as raw JSON rather than escaped
• src/pipeline/pass_parallel.c — same changes mirrored; version_tag threaded through parallel worker
• src/mcp/mcp.c — cbm_derive_version_from_path() helper, version param on index_repository, diff_versions tool registered and handler implemented