fix: Clear error messages with structured errors and suggestions

[whitmo]

Summary

• P0 Roadmap item: "Clear error messages - Every failure should tell the user what went wrong and how to fix it"
• Audited internal/cli/cli.go (~5500 lines) and internal/daemon/daemon.go for raw errors lacking user context
• Converted ~70 raw fmt.Errorf calls to use structured internal/errors constructors with categories, messages, and actionable suggestions
• Added 7 new error constructors and improved daemon socket response messages with recovery hints

Key improvements by failure path:

• Daemon not running: DaemonCommunicationFailed() with "multiclaude daemon start" suggestion (replaces "failed to X: %w (is daemon running?)")
• Agent/worker not found: Existing AgentNotFound() used more consistently, new AgentRestartFailed()
• Worktree issues: WorktreeCreationFailed() used in all paths, cleanup suggestions
• Tmux failures: TmuxOperationFailed() with manual cleanup commands
• Claude binary: ClaudeNotFound() and new ClaudeStartupFailed() with install link
• Validation: InvalidArgument() for all flag validation (--mq-track, --ps-enabled, etc.)

New error constructors:

ClaudeStartupFailed, DaemonAlreadyRunning, AgentRestartFailed, PromptWriteFailed, LogFileNotFound, WorkspaceAlreadyExists, RepoAlreadyInitialized

Test plan

• All existing tests pass (go test ./...)
• New constructor tests added and passing
• Build compiles cleanly
• Error formatting verified via existing Format() tests

🤖 Generated with Claude Code

[whitmo]

Triage Review

Priority: P0 (Clear error messages - roadmap item)
Build: Pass
Tests: Pass (errors + cli packages)
Merge conflicts: Conflicts with PR #360 (cli.go) and PR #362 (daemon.go)
Roadmap alignment: Directly addresses P0 "Clear error messages" item

Changes:

• Converts ~30 raw fmt.Errorf calls to structured errors.* constructors
• Adds new error constructors: RepoAlreadyInitialized, InvalidArgument, PromptWriteFailed, ClaudeNotFound, ClaudeStartupFailed
• Each error includes actionable suggestions for the user

Recommendation: Merge AFTER #360 and #362 to avoid conflicts, then rebase.

[whitmo]

Local CI Verification (2026-03-12)

Check	Result
go build	PASS
go vet	PASS
go test ./...	PASS (all packages)

CI Status: No GitHub Actions checks are running — this is expected for first-time fork PRs. GitHub requires a maintainer to approve workflow runs for PRs from forks.

Branch is rebased on upstream/main (0 commits behind). Ready for maintainer review and CI approval.