Actionable error messages for git init sidecar sync failures

[schurchleycci]

Summary

• Fixes Sync fails with "An unknown error occurred" when git remote origin is not configured #349: when origin remote is not configured, chunk sidecar sync/setup now surfaces a typed NoOriginRemoteError with the message "Git remote "origin" is required for sidecar sync." and suggestion "Run: git remote add origin <url>", instead of the generic "An unknown error occurred"
• Fixes "Push your branch" suggestion does not resolve sync failure on git-init projects #350: when the upstream tracking branch is set but origin/HEAD is missing (common after git init + git push -u without git fetch), MergeBase() now detects the specific case and surfaces ErrNoOriginHEAD with the suggestion "Run: git fetch origin && git remote set-head origin -a", instead of repeating the "Push your branch" suggestion that was already followed

Changes

• internal/gitutil/gitutil.go — add ErrNoOriginHEAD sentinel; MergeBase() checks whether the upstream is set before returning the generic no-origin-HEAD error, and wraps ErrNoOriginHEAD when it is
• internal/sidecar/errors.go — add NoOriginRemoteError type (mirrors existing RemoteBaseError pattern)
• internal/sidecar/sync.go — wrap DetectOrgAndRepo failures as NoOriginRemoteError instead of a plain fmt.Errorf
• internal/cmd/sidecar.go — handle NoOriginRemoteError and ErrNoOriginHEAD in both newSidecarSyncCmd and sidecarSetupSync error paths

Note on PR #339

This PR is complementary to #339 (sync redesign). #339 renames the sync entry point and changes MergeBase()'s fallback command but does not address error surfacing. If #339 merges first, two small adaptations are needed: apply NoOriginRemoteError wrapping to both DetectOrgAndRepo calls in #339's new Bootstrap/Sync functions, and move the ErrNoOriginHEAD check onto the git merge-base HEAD origin/HEAD fallback that #339 introduces.

Test plan

• All unit and acceptance tests pass (task test)
• Lint clean (task lint)

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