feat(notebooks): add cells append command for cell-level operations

[majorgreys]

Summary

• Adds notebooks cells append subcommand that appends individual cells to an existing notebook
• Supports three cell types via simplified JSON: markdown, metric, and logs

Changes

• cmd/notebooks.go: Add cells subcommand group with append command, simplified cell types, cell builders for markdown/metric/logs, time parser, and append loop with partial failure handling
• cmd/notebooks_test.go: Add 15 new tests covering command structure, cell building, time parsing, validation errors, and runtime behavior
• docs/COMMANDS.md: Update notebooks entry to reflect new capabilities

Usage

# Append a markdown cell
echo '{"cells":[{"type":"markdown","data":"# Investigation Notes\nService latency spike observed."}]}' | \
  pup notebooks cells append 12345 --body -

# Append a metric graph from file
cat <<JSON > cells.json
{"cells":[
  {"type":"metric","data":"avg:system.cpu.user{*}","title":"CPU Usage",
   "start":"2025-02-14T00:00:00Z","end":"2025-02-14T01:00:00Z"},
  {"type":"logs","data":"service:api status:error","title":"API Errors"}
]}
JSON
pup notebooks cells append 12345 --body @cells.json

Design Notes

• All cells are validated before any API calls to prevent partial failures
• Metric cells default to timeseries line graphs; logs cells default to log stream with timestamp/host/service/message columns

Test plan

• go build ./... passes
• go test ./cmd/ -run "TestNotebooks|TestBuildCell|TestParseCellTimes" -v — 25 tests pass
• go test ./... -race — all tests pass
• go vet ./... — clean

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