fix: diagnose update path conflicts

[hehanlin1996]

Summary

Improve lark-cli update diagnostics for install/update instability reported around multiple installation paths and post-update binary verification failures.

Changes

• Add PATH scanning for multiple lark-cli executables and surface a path_conflict block in JSON output plus a human-readable warning in update --check, manual-update, update-success, and verification-failure paths.
• Expand binary verification failure hints to explain stale npm/temp-path cases and tell users to check lark-cli --version, clear shell command cache, and inspect duplicate installs.
• Cover the new diagnostics with TDD unit tests for human output, JSON output, and verification hint text.

Test Plan

• go test ./cmd/update -run 'TestUpdateCheck_(HumanWarnsOnMultiplePathCandidates|JSONIncludesPathConflict)$' -count=1 (red before implementation, green after)
• go test ./cmd/update -run 'TestVerificationFailureHintExplainsStaleNpmPath|TestUpdateNpmVerifyFail_JSON_NoRestoreHintWhenBackupUnavailable|TestUpdateCheck_(HumanWarnsOnMultiplePathCandidates|JSONIncludesPathConflict)$' -count=1
• go test -race -count=1 ./cmd/update ./internal/selfupdate
• go vet ./cmd/update ./internal/selfupdate
• git diff --check
• go run . update --help
• make unit-test currently fails in unrelated shortcuts/base MIME expectation: large-report.bin resolves to application/x-sega-cd-rom instead of expected application/octet-stream on this runner.

Related Issues

• Feedback cluster: C2-INSTALL-UPDATE (BASE-20260515-037, BASE-20260515-058)