chore: bump golangci-lint and apply formatting

[retr0h]

Summary

• Bump golangci-lint v2.11.4 → v2.12.2 (via go get -tool) and tidy go.sum
• Bump charmbracelet/x/cellbuf to v0.0.15 to fix an ansi API mismatch (old cellbuf was calling Italic()/Strikethrough()/etc. with no args, but the newer ansi v0.11.7 pulled in by golangci-lint and charm.land/lipgloss/v2 requires a bool argument)
• Replace reflect.Ptr with reflect.Pointer in internal/validation/validation.go (newer govet flags the legacy alias)
• Re-run gofumpt + golines across the tree

Test plan

• `just go::vet` — 0 issues
• `just go::unit` — all packages pass
• CI green

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@           Coverage Diff            @@
##             main     #393    +/-   ##
========================================
  Coverage   99.91%   99.91%            
========================================
  Files         484      484            
  Lines       24721    24935   +214     
========================================
+ Hits        24699    24913   +214     
  Misses         14       14            
  Partials        8        8            

Files with missing lines	Coverage Δ
internal/agent/handler.go	100.00% <100.00%> (ø)
internal/agent/preflight.go	100.00% <100.00%> (ø)
internal/agent/processor.go	100.00% <100.00%> (ø)
internal/agent/processor_certificate.go	100.00% <100.00%> (ø)
internal/agent/processor_interface.go	100.00% <100.00%> (ø)
internal/agent/processor_network.go	100.00% <100.00%> (ø)
internal/agent/processor_ntp.go	100.00% <100.00%> (ø)
internal/agent/processor_route.go	100.00% <100.00%> (ø)
internal/agent/processor_schedule.go	100.00% <100.00%> (ø)
internal/agent/processor_service.go	100.00% <100.00%> (ø)
... and 120 more

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