Walk Track: Extend debug-level timing instrumentation to search, node_list, ssh

[nikhil2611]

Context

Walk Ex9 added Chef::Log.debug("op=knife_...")) timing to knife status and knife node show. Three high-usage commands remain uninstrumented: search, node list, and ssh.

Code Paths

• lib/chef/knife/search.rb — wrap search call with CLOCK_MONOTONIC
• lib/chef/knife/node_list.rb — wrap list query
• lib/chef/knife/ssh.rb — wrap SSH session setup

Acceptance Criteria

• Each command emits Chef::Log.debug("op=knife_<cmd> status=ok elapsed_ms=N") after main I/O
• One new spec per command asserting Chef::Log.debug called with the timing pattern
• cookstyle passes; existing specs remain green
• ai-track-docs/observability.md updated

Dependencies

Walk Ex9 timing pattern (merged). No blocking deps.

Size

Small — one PR covering all three commands.