feat(ssh): add ssh command with output truncation

[lgbarn]

Summary

• Add rtk ssh command that passes through to ssh and compacts output
• Strips blank lines and caps stdout at 80 lines, stderr at 5 lines
• Add rewrite rule so ssh host cmd → rtk ssh host cmd via hook

Closes #783
Closes #333

Motivation

rtk discover showed 326 unhandled ssh commands over 30 days. SSH remote commands frequently produce verbose output (docker logs, journal dumps, systemctl status) that wastes tokens when piped back to an LLM.

Changes

File	Change
src/cmds/cloud/ssh_cmd.rs	New: passthrough handler with blank-line stripping and 80-line cap
src/discover/rules.rs	Add ssh rule (pattern ^ssh\s+\S+, category Infra, 50% est. savings)
src/main.rs	Add Ssh command variant, dispatch, and hook registration

Design decisions

• Passthrough with truncation rather than parsing: SSH output is unpredictable (could be JSON, logs, status tables) so we just cap lines rather than trying to parse structure
• 80-line stdout cap: matches the typical useful output length for LLM context; anything beyond is noise
• 5-line stderr cap: connection warnings and auth messages are useful; full error dumps are not

Test plan

• cargo test ssh — 4 new tests pass (truncation, short output, blank lines, empty)
• cargo test — 1257 passed, 3 ignored, 0 failed
• rtk rewrite "ssh pi@rpi docker ps" → rtk ssh pi@rpi docker ps

[CLAassistant]

All committers have signed the CLA.