feat(user): add SSH key management

[retr0h]

Summary

• Add SSH authorized key management to the existing user provider
• Three new operations: list keys, add key, remove key by fingerprint
• Operates on ~/.ssh/authorized_keys for any system user
• Nested under existing user endpoints: /node/{hostname}/user/{name}/ssh-key
• Reuses existing user:read/user:write permissions — no new permissions
• No state tracking — OSAPI manages any key regardless of who added it
• Idempotent: AddKey returns changed: false if fingerprint already present

Test plan

• Unit tests pass (just go::unit)
• Lint clean (just go::vet)
• 100% coverage on all new code
• Verify on Debian host: osapi client node user ssh-key list --target _any --name root
• Verify add: osapi client node user ssh-key add --target _any --name testuser --key "ssh-ed25519 AAAA... test"
• Verify remove by fingerprint
• Verify idempotent add (same key twice → changed: false)
• Verify --json output for all commands
• Verify broadcast: --target _all

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@           Coverage Diff            @@
##             main     #338    +/-   ##
========================================
  Coverage   99.89%   99.90%            
========================================
  Files         410      415     +5     
  Lines       18979    19569   +590     
========================================
+ Hits        18960    19550   +590     
  Misses         11       11            
  Partials        8        8            

Files with missing lines	Coverage Δ
internal/agent/processor.go	100.00% <100.00%> (ø)
internal/agent/processor_ssh_key.go	100.00% <100.00%> (ø)
...nternal/controller/api/node/user/ssh_key_create.go	100.00% <100.00%> (ø)
...nternal/controller/api/node/user/ssh_key_delete.go	100.00% <100.00%> (ø)
...ernal/controller/api/node/user/ssh_key_list_get.go	100.00% <100.00%> (ø)
internal/provider/node/user/darwin.go	100.00% <100.00%> (ø)
internal/provider/node/user/debian_ssh_key.go	100.00% <100.00%> (ø)
internal/provider/node/user/linux.go	100.00% <100.00%> (ø)
pkg/sdk/client/user.go	100.00% <100.00%> (ø)
pkg/sdk/client/user_types.go	100.00% <100.00%> (ø)

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 84198cf...ebf5316. Read the comment docs.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.