feat(notebooks): add append-only edit command

[mbldatadog]

Why this is needed

This command is required to ship the LLMObs skills (/eval-trace-rca, /eval-bootstrap, /experiment-analyzer) on top of pup.

Those skills currently call Datadog MCP notebook tools (create_datadog_notebook, edit_datadog_notebook) to write persistent analysis artifacts. When the skills run through pup instead of the MCP server, they need an equivalent pup command. notebooks create already exists; this PR adds the missing append-only variant.

Skills that break without this

/eval-trace-rca and /llm-obs-trace-rca — Phase 6 exports the root cause analysis report as a formatted Datadog notebook. Without notebooks edit, the skill produces console-only output with no shareable artifact. It also breaks the downstream chaining workflow described below.

/eval-bootstrap — After bootstrapping an evaluator suite, the skill detects whether a prior RCA notebook was created in the same session (by matching the notebook URL) and appends the bootstrapped evaluator summary to that notebook rather than creating a new one. This is the append-only pattern: the skill calls edit_datadog_notebook with append_only=true to preserve the existing RCA content and add new cells alongside it. Without notebooks edit, this entire chained workflow — /eval-trace-rca → creates notebook → /eval-bootstrap → appends to same notebook — is broken at the second step.

/experiment-analyzer — Phase 5 optionally exports the analysis report to a Datadog notebook. Without notebooks edit (or notebooks create), the skill falls back to chat-only output with no persistent artifact.

What breaks end-to-end

The most impactful breakage is the RCA → bootstrap chain:

/eval-trace-rca          # creates notebook with RCA findings
    ↓ notebook URL passed to next skill
/eval-bootstrap          # appends evaluator suite to same notebook
    ↓ broken without notebooks edit

Users lose the ability to produce a single unified notebook that documents both the root cause diagnosis and the evaluator suite that addresses it.

Summary

Adds pup notebooks edit <id> --file <cells.json> as an append-only alternative to notebooks update. Instead of replacing the entire notebook, it fetches the current notebook, appends the provided cells, and writes back — preventing accidental clobber of existing content.

Changes

• src/commands/notebooks.rs — new edit() function (read-modify-write via typed API)
• src/main.rs — NotebookActions::Edit variant and routing

Interface

# --file contains a JSON array of cell objects to append (not the full notebook)
pup notebooks edit <notebook_id> --file cells.json

Example cells file:

[
  {
    "attributes": {
      "definition": {
        "type": "markdown",
        "text": "## New Section\n\nContent here."
      }
    },
    "type": "notebook_cells"
  }
]

Testing

Automated

• cargo test — passing
• cargo clippy -- -D warnings — clean
• cargo fmt --check — clean

Manual smoke tests

cat > /tmp/cells.json << 'BODY'
[{"attributes": {"definition": {"type": "markdown", "text": "## pup smoke test\n\nAppended by pup notebooks edit — safe to delete."}}, "type": "notebook_cells"}]
BODY

• pup notebooks edit 13937752 --file /tmp/cells.json — cell appended to notebook, returns updated notebook (exit 0)
• pup notebooks edit 999999999 --file /tmp/cells.json — 404 "Notebook not found" on the initial fetch (exit 1)

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