feat: usability improvements from Nielsen heuristic audit

[sunapi386]

Summary

• Systematic audit against Jakob Nielsen's 10 usability heuristics identified gaps in user control (H3), discoverability (H6/H7), consistency (H4), and error recovery (H9)
• Adds confirmation prompts to destructive commands (stop, logout), shell completion support, global --no-color, --json output for key commands, and improved error messages with recovery hints

Changes

H3: User Control & Freedom

• citadel stop now prompts before stopping all services; --rm uses default-No confirmation
• citadel logout now prompts before disconnecting
• Both support --force / -f to skip prompts (for scripts/CI)
• citadel stop --dry-run previews what would be stopped

H6/H7: Discoverability & Efficiency

• New citadel completion [bash|zsh|fish|powershell] command via Cobra built-in generators
• --json flag on status, nodes, peers for machine-readable output
• JSON struct tags added to StatusData, GPUInfo, ServiceStatus, PeerInfo

H4: Consistency

• --no-color moved from status-only to global persistent flag in root.go
• Respects NO_COLOR environment variable (standard convention)

H9: Error Recovery

• Manifest "corrupt" error now says "Run 'citadel init' to regenerate"
• Service start failures hint "Run 'citadel logs ' for details"
• Docker compose errors suggest checking if Docker is running
• Container-not-found errors suggest citadel status

Test plan

• go build ./cmd/citadel/ compiles clean
• go test ./cmd/ ./internal/... all unit tests pass
• Manual: citadel stop shows confirmation, --force skips it, --dry-run previews
• Manual: citadel logout shows confirmation, --force skips it
• Manual: citadel completion bash | head outputs completion script
• Manual: citadel status --json | jq . outputs valid JSON
• Manual: citadel --no-color status disables colors globally
• Manual: NO_COLOR=1 citadel status disables colors via env

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