feat(network): add interface and route management via Netplan

[retr0h]

Summary

Full CRUD for network interface configuration and route management via Netplan drop-in files, following the direct-write provider pattern.

• Interface CRUD: list, get, create, update, delete — generates /etc/netplan/osapi-{name}.yaml
• Route CRUD: list, get, create, update, delete — generates /etc/netplan/osapi-{name}-routes.yaml
• DNS delete: remove managed DNS Netplan config
• Safety: default route (0.0.0.0/0, ::/0) protection blocks accidental deletion
• Validation: netplan generate validates merged config before netplan apply; rollback on failure
• Managed flag: interface and route lists show managed: true/false; interface list includes down/unlinked interfaces with osapi config files
• Idempotent Create/Delete: all providers (cron, certificate, service, NTP, sysctl, interface, route) now return changed: false instead of erroring when creating an already-managed resource or deleting an absent one
• Netplan deep-merges separate files per interface (config, routes, DNS)
• Standardized error messages to "already managed"/"not managed" across all providers

Layers implemented

• Provider: internal/provider/network/netplan/ (interface + route CRUD, Debian impl, platform stubs)
• Agent: processors for interface.* and route.* operations
• API: 11 new handler endpoints under /node/{hostname}/network/
• SDK: InterfaceService and RouteService with typed methods
• CLI: osapi client node network interface and route command trees
• Docs: feature page, 13 CLI reference pages, SDK docs, examples, cross-references
• CLAUDE.md: added Idempotency (MANDATORY) section for provider development

Test plan

• Provider: interface + route CRUD at 100% coverage
• Agent: processor tests for all operations
• Handlers: unit + HTTP validation + RBAC tests
• SDK: all methods + type conversion tests
• Idempotent Create/Delete tested across all 7 providers
• Build clean, lint clean

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff            @@
##             main     #349     +/-   ##
=========================================
  Coverage   99.91%   99.92%             
=========================================
  Files         443      469     +26     
  Lines       21606    23814   +2208     
=========================================
+ Hits        21587    23795   +2208     
  Misses         11       11             
  Partials        8        8             

Files with missing lines	Coverage Δ
internal/agent/preflight.go	100.00% <ø> (ø)
internal/agent/processor_interface.go	100.00% <100.00%> (ø)
internal/agent/processor_network.go	100.00% <100.00%> (ø)
internal/agent/processor_route.go	100.00% <100.00%> (ø)
internal/controller/api/node/network/dns_delete.go	100.00% <100.00%> (ø)
internal/controller/api/node/network/dns_get.go	100.00% <100.00%> (ø)
internal/controller/api/node/network/dns_put.go	100.00% <100.00%> (ø)
...ntroller/api/node/network/interface_create_post.go	100.00% <100.00%> (ø)
...al/controller/api/node/network/interface_delete.go	100.00% <100.00%> (ø)
...ernal/controller/api/node/network/interface_get.go	100.00% <ø> (ø)
... and 81 more

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