feat: improve error messages with structured errors and suggestions

[aronchick]

Summary

P0 Roadmap item: Clear error messages - Ensure every failure tells the user what went wrong and how to fix it.

• Add 17 new error constructors to internal/errors for common failure modes
• Update ~50 error handling locations in CLI to use structured errors with suggestions
• Replace raw fmt.Errorf calls with CLIError that provides actionable guidance

Before/After Examples

Before:

Error: failed to register worker: connection refused 

After:

Error: failed to register worker with daemon: connection refused Try: multiclaude daemon status 

Before:

Error: repository 'myrepo' is already initialized Use 'multiclaude repo rm myrepo' to remove it first, or choose a different name 

After:

Configuration error: repository 'myrepo' is already initialized Try: multiclaude repo rm myrepo # to remove and re-init 

New Error Types

Error Constructor	Use Case	Suggestion
RepoAlreadyExists	Repo init when already tracked	multiclaude repo rm
DirectoryAlreadyExists	Clone target exists	Remove manually
WorkspaceAlreadyExists	Workspace name taken	multiclaude workspace list
InvalidWorkspaceName	Bad workspace name	Naming rules
LogFileNotFound	Missing log file	Agent may not have started
InvalidDuration	Bad duration format	Format examples
NoDefaultRepo	Can't determine repo	Use --repo or repo use
StateLoadFailed	Corrupt state	multiclaude repair
SessionIDGenerationFailed	UUID failure	Try again
PromptWriteFailed	Can't write prompt	Check disk/permissions
ClaudeStartFailed	Claude won't start	Check claude binary/tmux
AgentRegistrationFailed	Daemon registration	multiclaude daemon status
WorktreeCleanupNeeded	Stale worktree	git worktree remove
TmuxWindowCleanupNeeded	Stale window	tmux kill-window
TmuxSessionCleanupNeeded	Stale session	tmux kill-session
WorkerNotFound	Worker doesn't exist	multiclaude worker list
AgentNoSessionID	Missing session ID	Remove and recreate

Test plan

• All internal package tests pass (go test ./internal/...)
• All pkg tests pass (go test ./pkg/...)
• Build succeeds (go build ./cmd/multiclaude)
• Manual verification of error messages in common failure scenarios

🤖 Generated with Claude Code

[aronchick]

✅ Merge Queue: READY TO MERGE

• CI: All 6 checks passing
• Merge status: MERGEABLE/CLEAN
• Reviews: None blocking

This PR is ready for maintainer merge.