Mark truncated lists in Orbit context llm format

Surface truncated lists in the llm format of the Orbit context endpoint. When ContextService cuts a list, the text output now adds a truncated= line naming it, so an LLM reader knows the list is incomplete.

Agent context

• Marker shape: one truncated=reviewers,changed_files line per entity, right after the scalar fields so the reader learns up front that lists are partial. It matches the key=value lines and the truncated=true header in the grep output.
• Entity-level names (reviewers, assignees, labels, linked items, changed_files, failed_jobs, discussions) come from summary[:truncated]. A merge request's failed job carries its own truncated=failed_tests line under the job, because that cut lives on the job hash.
• Both lines are omitted when nothing is cut.
• Docs: added a sentence to doc/api/orbit.md.
• Specs: extended context_llm_formatter_spec.rb; RSpec run pending a Caproni slot. Rubocop is clean.
• No project issue exists; labels and milestone follow !262223 (merged). No VERSION bump, since the change is additive.