fix(multipass): use key=value format for multipass set commands

[laolusrael]

Summary

Fixes PUT /api/instances/{name}/resources and all other multipass set calls failing because key and value were passed as separate arguments instead of key=value.

Root Cause

Multipass set command requires key=value as a single argument:

# Wrong (what we were doing)
multipass set local.claw.memory 4.0G

# Correct
multipass set local.claw.memory=4.0G

This caused the error:

Need exactly one key-value pair (in <key>=<value> form).

Changes

1. Fix SetInstanceResources — CPU, memory, and disk now use key=value format
2. Fix snapshot comment setter — also changed to key=value format
3. Accept decimal values — parseMemoryString uses ParseFloat to handle "4.0G"
4. Capture stderr — CombinedOutput() surfaces real multipass error messages
5. Skip unchanged values — only calls multipass set for values that actually changed
6. Update docs — corrected multipass set syntax in API reference

Files Changed

• api/internal/multipass/client.go
• docs/MULTIPASS_API_REFERENCE.md

Test Results

• go test -tags ci ./... — Pass