feat(issues): add discussion summary commands for decisions and open questions

[iamfj]

What problem does this solve?

Issue discussions often contain the real implementation context: decisions, tradeoffs, open questions, and corrections to the original description. Today, Linearis can expose raw discussion data, but it does not help users or agents extract the signal.

This creates friction:

• long threads are expensive to re-read
• important decisions are easy to miss
• contradictions between description and comments are hard to detect quickly
• agents cannot easily consume a compact “what changed in discussion?” view

Proposed solution

Add summary-oriented issue discussion commands:

linearis issues discussion-summary <issue>
linearis issues decisions <issue>
linearis issues open-questions <issue>

Suggested behavior:

• summarize comment threads into structured JSON
• identify likely decisions, unresolved questions, and recent discussion changes
• remain explicitly opt-in and machine-oriented

Example output:

{
  "issue": {
    "id": "issue-id",
    "identifier": "ENG-123"
  },
  "discussionSummary": {
    "decisions": [
      "Ship the simpler first version before adding more depth"
    ],
    "openQuestions": [
      "Whether onboarding analytics should be included in v1"
    ],
    "notableChanges": [
      "Discussion clarified that the issue scope should stay focused on the core flow"
    ]
  }
}

Alternatives considered

• Leave summarization to external agents entirely. That misses an opportunity for Linearis to provide a stable, structured collaboration primitive.
• Return free-form text only. Less reliable for automation than structured JSON.

Primary use case

LLM agent integration

Additional context

Related: #144, #145, #146

Acceptance criteria:

• at least one summary-oriented issue discussion command is implemented
• output is structured JSON rather than plain prose only
• summaries are derived from issue discussion, not just issue description
• tests cover stable output shape
• help text and usage docs are updated

[linear]

LIN-130